Chromosomal abnormalities are the most frequent cause of first trimester spontaneous abortions (SA). During the period September 1989 through May 1996 we have cytogenetically studied 640 embryonic tissue samples obtained from cases of SA. Of these, 609 samples (95.1%) were successfully karyotyped. An abnormal karyotype was observed in 388 cases (63.7%). The sex ratio (XY/XX) was 1.03. There was no difference of this ratio between cytogenetically normal and abnormal embryos. The most frequent abnormalities detected were autosomal trisomies (239/388 [61.6%]) followed by triploidy (62 cases, 16%), monosomies (41 cases, 10.6%) and tetraploidy (17 cases, 4.4%). The single most common anomaly observed was trisomy 16 (73 cases, 18.8%). The frequency of trisomies, with the exception trisomy 16, was related with advanced maternal age.