Most studies on human sperm chromosomes from normal men involve the heterologous fertilization of zona free hamster eggs by unselected human sperm. In this work, we have performed cytogenetic studies of highly motile sperm, selected by a swim-up method. A total of 505 motile human sperm complements from three normal donors was analysed. The total frequency of sperm with chromosomal abnormalities (10.9%; 6.9% structural aberrations and 4.0% aneuploidy) and the sex ratio (50.4% X:49.6% Y) were similar to those obtained from whole semen samples. Our results indicate that the selection of motile sperm does not imply chromosomal selection. © 1992 Springer-Verlag.