A woman with recurrent Paget's disease of the vulva developed acute graft-versus-host disease (GVHD) 12 days after radical surgery and massive blood transfusion. Molecular diagnosis of lymphocyte chimerism in the peripheral blood was made by polymerase chain reaction (PCR) directed against a Y chromosome-specific sex-determining region Y (SRY) gene. PCR with skin biopsy after onset of GVHD also revealed infiltration of SRY-positive donor lymphocytes. The diagnosis was confirmed by HLA-DNA typing with PCR-sequence-specific oligonucleotide that revealed the presence of complex HLA-DR chimerism in the peripheral lymphocytes collected after onset of GVHD. The use of SRY-directed PCR is a rapid technique for the early diagnosis of acute posttransfusion GVHD in female patients.