We have previously shown that after 80% Percoll centrifugation there is an overall 2.7-fold reduction of sperm with chromosomal disomies and diploidies (3.2-fold and 2.0-fold respectively), and of sperm with diminished maturity as detected by cytoplasmic retention. The relationship between disomies and immature sperm was r = 0.7, suggesting that disomy primarily originates in immature sperm. In the present work we studied the efficacy of the swim-up method in elimination of sperm with diminished maturity and with chromosomal aberrations in the swim-up sperm fractions of 10 patients (sperm concentration: 20 +/- 3.9 x 10(6)/ml, range 8.9-45.5; sperm motility: 45.2 +/- 2.4, all mean +/- SEM). The validity of the study was enhanced by assessing each sperm fraction with three-colour (X, Y and 17; 5000 sperm) and two-colour (10 and 11; 5000 sperm) chromosome probes using fluorescence in-situ hybridization (FISH). Thus, in each sample 10 000 sperm were evaluated. The incidence of diminished maturity sperm was assessed with creatine kinase immunocytochemistry. In the swim-up fractions there was a reduction in the frequencies of disomic sperm, whether considering the sex chromosomes (1.4-fold) or the three autosomal chromosomes (1.5-fold based on the aggregate frequencies of disomy 10, 11 and 17). There was also a 1.5-fold reduction in diminished maturity sperm, indicating a relationship between the proportion of immature sperm and chromosomal aneuploidies (r = 0.46, P < 0.05, n = 20). Diploid sperm were reduced at a 2.7-fold rate, whether assessed with two- or three-colour FISH. There was a slight increase in the X/Y ratios. Swim-up reduces the proportion of sperm with chromosomal aberrations and of sperm with diminished maturity. When compared with the results of the previous study with gradient centrifugation performed on semen samples with similar quality, the efficacy after swim-up is lower for disomies and higher for diploidies than that of gradient centrifugation.