Two female sibling cases, who were born to a CD36 deficient mother, were presented with Coombs' test-negative hydrops. The alloimmune anti-CD36 (Nak(a)) antibody was accidentally found in the mother's serum after an episode of anaphylactic shock with thrombocytopenia, which occurred in an individual receiving fresh frozen plasma prepared from the mother's donated blood. The mother was then diagnosed as having type II CD36 deficiency, lacking CD36 on both platelets and monocytes, while both of her daughters were CD36 positive. Analyses of the CD36 gene revealed that the mother was a compound heterozygote for the CD36 gene mutation with a novel C --> T transition at nt 1366 in exon 12, corresponding to Arg386Trp, and a known 12bp deletion at nt 1438-1449 in exon 13. On the other hand, both patients, who showed half the normal level of CD36 on platelets and monocytes, were heterozygote with one mutation at Arg386Trp. The anti-CD36 antibody in the mother seemed to be responsible for the hydrops fetalis observed in her daughters, because the IgG isolated from the mother's serum showed suppressive effects on the CFU-E colony formation of CD34+ cells from a control donor. This is the first case report of hydrops fetalis caused by an alloimmune anti-CD36 antibody.