The aim of this study was to determine the relationship between sperm morphology and fertilization rates in vitro. Semen samples were obtained from 50 couples undergoing IVF treatment. Sperm morphology was classified by strict criteria (Tygerberg) according to head, midpiece and tail defects in neat semen and after sperm selection by Percoll gradient centrifugation. Percoll preparation significantly increased the percentage of sperm with normal morphology from 13 to 20%. However, the greatest single regression coefficient was observed with the percentage of sperm with tail defects and correlated negatively with fertilization rates in vitro both before and after Percoll preparation. Therefore, tail morphology may be of value as a prognostic factor in assisted conception both before and after Percoll preparation.