Cytogenetic analysis of 750 spontaneous abortions with the direct-preparation method of chorionic villi and its implications for studying genetic causes of pregnancy wastage. Altogether, 750 cases of spontaneous abortion between the fifth and 25th week of gestation were analyzed cytogenetically by the direct-preparation method using chorionic villi. The majority of cases (68%) were derived from early abortions before the 12th week of gestation. The frequency of abnormal karyotypes was 50.1%; trisomy was predominant (62.1%), followed by triploidy (12.4%), monosomy X (10.5%), tetraploidy (9.2%), and structural chromosome anomalies (4.7%). Among trisomies, chromosomes 16 (21.8%), 22 (17.9%), and 21 (10.0%) were prevalent. The frequency of chromosomally abnormal abortions increased with maternal age but only because of an increase of trisomy. Polyploidy and monosomy X, however, decreased. Mean maternal age was significantly increased for trisomies 16, 21, and 22 and was highest for trisomies 18 and 20. The results obtained are within the range of variability reported earlier from tissue culture-type studies. A consistent feature during our study is the excess of females in chromosomally normal abortions (male:female sex ratio 0.71). According to the methodology applied, maternal cell contamination and undetected 46,XX molar samples cannot have influenced the sex ratio. However, a bias introduced by social status or maternal age cannot be excluded. With the more rapid and convenient direct preparation of chorionic villi, reliable cytogenetic data on causes of spontaneous abortions can be obtained. 