This past week marked the 500th anniversary of one of the most horrific, and least remembered, massacres of Jews ever to have taken place on the streets of a major European capital. For three days beginning on April 19, 1506, a bloodthirsty mob goaded on by Dominican preachers roamed throughout Lisbon, the Portuguese capital, hunting down their victims and butchering them with unrestrained glee.

Their prey were the Cristaos Novos ("New Christians"), the thousands of Portuguese Jews who had been forcibly converted to Catholicism by order of King Manuel just a decade before. Some were beaten to death, others were beheaded. Many more were dragged through the streets and tossed alive into burning pyres that had been erected at the Rossio, Lisbon's main square. Anyone who looked Jewish, or was known to have had dealings with Jews, quickly found themselves a target of the murderous throng as well.

The orgy of violence reportedly began after one of the "New Christians" expressed doubt about a miracle that allegedly had taken place in a local church. He was immediately taken outside and butchered, along with his brother.

Then, according to a contemporary account written by Solomon Ibn Verga, a number of Dominican friars "went to the main avenue of the city and proclaimed: 'Anyone who kills the seed of Israel shall be granted one hundred days of absolution for the world to come!'"

The rabble took its cue, and proceeded to break into Jewish homes across the city and commit atrocities, which included tossing pregnant women out of windows onto spears held aloft by the crowd at ground level.

Aware of the mayhem taking place in the city, German, Dutch and French sailors based on ships in Lisbon's harbor are said to have come ashore - not in order to stop the bloodshed, but to join in on it instead.

All told, it is estimated that 2,000 to 4,000 Jews were slain in the massacre, underlining the hostility which they continued to face even after being compelled to abandon their faith and join the Church.

BUT DESPITE its ghastly nature, and the terrible toll it claimed in life and limb, the Lisbon massacre has been all but forgotten by much of the Jewish world, with few even aware that it took place.

Sure, prominent historians such as Heinrich Graetz and Meyer Kayserling wrote about it, as has Columbia University's Yosef Haim Yerushalmi. But outside of academic circles, the entire incident has been all but overlooked.

And while members of Lisbon's Jewish community held a small memorial ceremony last week, virtually nothing was done outside of Portugal to recall this dark moment in our people's history.

So the question remains: why has the Lisbon massacre, whose ferocity foreshadowed subsequent anti-Jewish carnage in Europe, been so thoroughly overlooked? And, no less crucial, does it really matter?

Of course, for a people who have experienced so much horror in the intervening centuries, it is only somewhat natural that our collective memory has grown dimmer with each additional blow that we suffered. To commemorate every massacre and pogrom, every expulsion and blood libel, would fill the calendar to overflowing with dates of sadness and despair.

Moreover, the proximity of the Holocaust, its unparalleled uniqueness, and the vast devastation that it wrought, clearly overshadow all that came before it.

Nonetheless, I don't think that excuses us from our responsibility to bear witness to our people's past. If we allow events such as the Lisbon massacre to fade into obscurity, we will be committing an unforgivable act of historical injustice, not only to those who were killed, but also to ourselves.

It is precisely by keeping alive the memory of these events that we gain a better understanding of our place both in the past and the present, as well as in the future. If we disregard that which came before us, we ultimately deny ourselves the opportunity to learn from history and to absorb its lessons.

In Zakhor, his classic work on Jewish history and Jewish memory, Yosef Haim Yerushalmi writes that, "hardly any Jew today is without some Jewish past. Total amnesia is still relatively rare. The choice for Jews, as for non-Jews, is not whether or not to have a past, but rather - what kind of past shall one have."

I for one choose to remember.

The writer is chairman of Shavei Israel, a Jerusalem-based group which assists "lost Jews" seeking to return to the Jewish people. www.shavei.org