To determine whether the characteristics of Percoll-selected spermatozoa are more predictive of in-vitro fertilization (IVF) results than are those of native semen, 118 semen samples from patients undergoing an IVF attempt were studied. Motility, using computer-assisted sperm analysis, and morphology were recorded before and after sperm selection on a Percoll gradient. Percoll selection increased the percentage of morphologically normal spermatozoa (58.2 +/- 19.9% versus 47.9 +/- 17.9%; p < 0.0001). This increase concerned almost all abnormalities and especially abnormalities of the midpiece (cytoplasmic droplets and bent tails). However, morphology of spermatozoa in the Percoll fraction had the same predictive value for IVF as did that for whole semen. Concerning motility, all parameters, except linearity, were improved by Percoll preparation, and their predictive value for IVF results was significantly higher in the Percoll fraction than in native semen. Therefore, it is concluded that, even if sperm characteristics are improved dramatically after Percoll selection, only motility analysis then gave more practical information than did analysis of sperm characteristics in native semen. This suggests that impairment of spermiogenesis, which presumably induces sperm abnormalities, is more important than are the actual characteristics of the spermatozoa.