Biochemical screening for Down syndrome (DS) is well established in the second trimester of pregnancy, but there is little information available on its value in the first trimester. This study describes our preliminary results with biochemical screening for DS in the first trimester of pregnancy in order to evaluate its efficacy at this time. Our study population, including 19 DS pregnancies, was evaluated using maternal serum levels of alpha-fetoprotein (AFP), beta-human chorionic gonadotropin (beta-hCG), and pregnancy-associated plasma protein A (PAPP-A). At a false positive rate (FPR) of 5 per cent, the detection rate (DR) for DS is 9 per cent for beta-hCG, 18 per cent for AFP, and 66 per cent for PAPP-A when considering these parameters individually. With different combinations of the analytes, the best detection rates are obtained with the association of PAPP-A and AFP (85 and 82 per cent DR for a 10 and 5 per cent FPR, respectively). Our data support the value of first-trimester biochemical screening for DS and that of PAPP-A as a single marker.