Isoimmune neonatal thrombocytopenia is an unusual problem with significant morbidity and mortality. Though it is often self-limited, it is potentially fatal due to hemorrhage. Specific therapy with compatible platelets is indicated. In previously unrecognized cases, platelet antigen and antibody studies delay therapy unacceptably. Therefore, maternal platelets are used, which also aid in establishing a therapeutic diagnosis. A case of isoimmune neonatal thrombocytopenia is reported, and the etiology, clinical and laboratory implications, and methods of management are discussed.