The occurrence of thrombocytopenia and leukopenia in association with penicillamine treatment has been described, as have been isolated cases of polycythemia. Thrombocytopenia, which can occur in 8% of rheumatoid arthritis patients treated with penicillamine, is usually readily reversible on drug withdrawal or after lowering the dosage. Leukopenia and bone marrow aplasia are rare in penicillamine treatment and can occur precipitously. Frequently monitoring of all penicillamine recipients is essential to prevent such potentially fatal side effects.