The cytogenetic findings in 20 experimental and 80 diagnostic cases of first trimester trophoblast biopsy are presented. All samples were obtained between the 8th and 13th week of gestation with the direct vision, trans-cervical technique. Except when fetal sexing because of X-linked disease was the issue, long-term culture with in situ preparation was the method routinely employed in processing the biopsies for cytogenetic analysis. In 78 of the 80 clinical cases and in all reported experimental cases we were successful in establishing a karyotype from the sampled tissue. Unbalanced karyotypes were found in two experimental and six clinical cases. Tetraploidy was found in one clinical case, but was not confirmed in subsequently sampled amniotic fluid cells. In another clinical case, we were unable to confirm in the aborted placenta the trisomy 18 found in the trophoblast biopsy. In the rest of the induced abortions in the clinical series, the karyotype arrived at prenatally has been confirmed, and the 27 babies so far born have been healthy and with phenotypic sex corresponding to the prenatal findings. Six women have miscarried after sampling; in one of these cases the fetus had the karyotype 47,XX, + 13.