We performed a prospective study on the incidence of thrombocytopenia (t-penia) and its immunological origin in unselected 26,275 mothers and 24,101 newborns. Platelet antibodies were examined by the platelet immunofluorescence test (PIFT) and the monoclonal antibody immobilisation of platelet antigens assay (MAIPA). T-penia (platelet count < 100 x 10(9)/I) was found in 124 (0.5%) mothers (in 0.04%) severe, <50x 10(9)/l) and in 116 (0.5%) newborns (in 0.15% severe); 90 (72.6%) and 112 (96.6%), respectively, were available for further studies. In both groups non-immune t-penia was diagnosed about 4.5 times more often than the immune t-penia. Among 90 mothers, t-penia was severe in 11.1%, antibodies were detected in 17.8%; both factors were not prognostic for delivering thrombocytopenic newborns. Among 112 babies, 21 were delivered by thrombocytopenic mothers and 91 by mothers with normal platelet count; among newborns with immune t-penia the proportion of alloimmune (NAIT) to autoimmune was equal (10 with NAIT, 10 with autoimmune, 4 of them born by mothers with hidden autoimmune t-penia). In 33% of the neonates t-penia was severe, most often among NAIT. In conclusion, although t-penia in mothers as well as in infants is not frequent and severe, and an immune origin not often found, the search for antibodies, in particular alloantibodies, should be done. Even if the serological results are not helpful at the moment, they can be of importance in subsequent pregnancy and for related pregnant women.