Flow cytometry was used to enumerate the lymphocyte subpopulations in fetal blood obtained by cordocentesis from eight trisomy 18 fetuses at 20-36 weeks' gestation. Compared with values in chromosomally normal fetuses, in trisomy 18 the mean T- and natural killer (NK) cell counts were significantly lower (t = -7.63, P < 0.001 and t = -3.58, P < 0.01, respectively); the mean B-cell count was not significantly different (t = -1.32). These findings demonstrate that in trisomy 18 there is abnormal intrauterine development of the immune system.