Neonatal thrombocytopenia (NNT) which is frequent in distressed newborns was uncommon in a non-selected population of neonates. The aim of this prospective study was to determine the frequency of NNT and, in confirmed NNT, to search for maternal antiplatelet antibodies with a monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. Among the 8388 newborns studied, 40 (0.5%, 95% CI 0.3-0.6) had confirmed NNT, which was severe (platelet count < 50 x 10[9]/l) in 10 cases (0.12%, 95% CI 0.05-0.20). Antiplatelet antibodies were detected in 10/31 studied mothers of thrombocytopenic newborns (32.3%): they were alloantibodies in five cases and autoantibodies in five other cases. Among these 10 newborns, seven had severe thrombocytopenia and four had bleeding complications. As controls, antiplatelet antibodies were also searched for in mothers of non-thrombocytopenic newborns: antiplatelet antibodies were present in 8.5% (95% CI 5.9-11.7) of thrombocytopenic mothers (n = 400) and 3.2% (95% CI 0.7-9.0) of non-thrombocytopenic mothers (n = 95). The difference was significant between the control groups and the group of mothers of thrombocytopenic newborns. In conclusion, our data indicate that an immune origin is frequent in NNT and should be looked for, particularly when the platelet count is < 50 x 10(9)/l.