The effectiveness of a seed disperser is assessed by the quantity (number of visits to fruiting trees and number of seeds dispersed) and quality of dispersal (seeds passed through the gut unharmed and how and where the seeds are dispersed). This is the first study to examine quantitative and qualitative aspects of seed dispersal by golden lion tamarins (Leontopithecus rosalia). The study was conducted from December 1998 to December 2000 and from April 2003 to March 2004 in the União Biological Reserve, Brazil. We marked 1,185 fruiting trees visited by tamarins and collected 1039 feces with seeds. About 76% of feces deposited by L. rosalia did not differ from the spatial distribution of plants of the same species. The mean time of gut retention was 1:14 h, and the mean distance of dispersal was 105 m. Given the effective role of L. rosalia as seed dispersers, their presence in the Atlantic forest is important for the regeneration of the forest.