High-dose chemotherapy with stem cell support is associated with a period of profound thrombocytopenia. During periods of thrombocytopenia, alloreactive antibodies can limit the effectiveness of platelet transfusion. In this institution, women undergoing high-dose chemotherapy with stem cell support for treatment of their ovarian cancer seemed to have a high incidence of panel reactive antibodies (PRA). To evaluate this further, a case-control study was performed comparing pretransplant PRA values in 22 consecutive women undergoing high-dose chemotherapy as intensification therapy for their ovarian cancer and a control group of 51 women receiving similar or identical therapy for their breast cancer. Parity was similar in both groups. Nonparametric t-test comparison of the two groups demonstrated a significantly higher PRA value in women with ovarian cancer compared to those with breast cancer (p = 0.032). Platelet transfusions were more common in women with ovarian cancer as compared to those with breast cancer (5.45/pt and 4.79/pt), although this difference did not reach statistical significance. This finding suggests that alloimmunization may be a more frequent problem in patients with ovarian cancer than in women with breast cancer.