Chromosome studies were performed on 288 consecutively collected early spontaneous abortuses and their parents and on 103 cases of threatened abor­tions by means of Q- and C-banding techniques. The frequency of chromosome anomalies was 61% in first trimester abortions and 55% in the entire series. In the parents the frequency of major chromosome anomalies and chromosome variants was 0.76% and 26.9%, respectively. The mean maternal age was increased in abortions with trisomy 9, 15 and 21. The origin of the extra chro­mosome or chromosome set was determined in trisomy 13 (1 case), trisomy 16 (11 cases) and in triploidy (9 cases). The recurrence risk following an abortion was 14.2% if there had been no previous abortions. If there had been previous abortions the frequency of a subsequent abortion was 45% in cases with a karyotypically normal index abortus and 20% in cases with a karyotypically abnormal index abortus. Of the mothers with threatened abortion, 43% proceeded to term, while 57% aborted. All infants had a normal karyotype whereas 56% of the abortuses were karyotypically abnormal.