The first American study on the incidence of gestational Rh0(D) sensitization is reported. The authors studied 3995 Rh0(D)-negative pregnancies from a population of 40,319 deliveries at Los Angeles-area Kaiser hospitals between 1976 and 1978. Among these were 61 Rh0(D)-sensitized pregnancies: 38 in women with prior sensitizations, 21 with gestational sensitizations, and 2 postpartum Rh0(D antigen) immune globulin (RhoGAM) failures. The incidence of gestational sensitization was 1.0% (21/2098). Cost-effectiveness of prophylactic RhoGAM post partum, prophylactic RhoGAM at 28 weeks' gestation, and Fetaldex testing post partum was analyzed. Prophylactic RhoGAM at 28 weeks' gestation was the most cost-effective method of further reduction of the Rh0(D) sensitization. All nonsensitized Rh0(D)-negative mothers who delivered Rh0(D)-positive fetuses received RhoGAM.