The purpose of this study was to investigate whether the multiple marker screening test (MMST) might be used as a screening test to detect chromosomal abnormalities during the first trimester. We performed MMSTs on stored serum samples from women who had undergone an early ammiocentesis during 1988-1993. Medians at each gestational age 11-136/7 weeks were calculated. A positive screening test was a Down Syndrome (DS) risk of ± 1:190 in the midtrimester. The results of the serum tests were compared to the fetal karyotype results and pregnancy outcome data. Three hundred eighteen samples were assayed. Two hundred seventy-five women (86% were ≤ age 35 at delivery. There were 76 (24% positive screening tests reflecting the large number of women ≤ age 35. Eight fetuses had abnormal karyotypes. The MMST identified 4/8 chromosome abnormalities, 2 of 2 (100% cases of Down syndrome, 1 of 1 case of trisomy 13, and 1 of 3 (33% sex chromosomal abnormalities. The detection/amniocentesis rate was 1/38 for DS and 1/19 overall. The MMST may prove a useful screening test for chromosomal abnormalities in the first trimester. Further prospective studies are needed to confirm this conclusion. © 1995 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.