Polish President Alexander Kwasniewski has been visiting Ukraine for the first-ever memorial ceremony to mark the anniversary of a notorious massacre during World War II.
The massacre is now being seen as a common tragedy
The visit follows a vote on Thursday, in which the two countries' parliaments approved a declaration aimed at resolving the highly sensitive dispute.
The massacres took place in 1943 and 1944 in Volyn (Wolynia) - which was then part of Poland - when nationalists on both sides killed tens of thousands of civilians.
The BBC's Adam Easton in Warsaw says Mr Kwasniewski, and his Ukrainian counterpart Leonid Kuchma, will be hoping the event will allow the countries to get over their painful past and look toward a brighter future.
Historians estimate that up to 100,000 Poles were killed by Ukrainian nationalists in the ethnic conflict.
As many as 20,000 Ukrainians are believed to have died in revenge attacks.
Members of the Ukrainian parliament, however, complained that the text of the declaration did not adequately acknowledge the Ukrainian deaths.
The declaration, which was drafted by a Polish-Ukrainian committee, described the event as a tragedy of the Polish people in its very first sentence.
That upset many Ukrainian parliamentarians, who noted that their country's victims did not get a mention until the end of a paragraph.
The two leaders are hoping their countries can now look to the future
The parliament went into recess while the speaker telephoned his Polish counterpart in an unsuccessful bid to amend the text.
In the event 227 members voted for the declaration, one more than the absolute majority needed for it to pass.
The Polish parliament passed the declaration, by 323 votes to 35.
Some MPs voted against it because it referred to the killings as ethnic cleansing and not genocide.
"There should not be any excuse for terror, violence and cruelty. The truth of those dramatic years is painful for everybody, but both Polish and Ukrainians should know about it," the statement said.
The Pope this week called for reconciliation between the two nations, saying that they should make "a thorough account of sins" and "look at each other with an eye for reconciliation".