The efficacy of thioguanine in the treatment of severe cases of psoriasis is demonstrated. This treatment is valuable in selected cases of severe psoriasis in whom other treatment is ineffective or impossible due to side effects. The effect of thioguanine on psoriasis lesions appears to run parallel with depression of the bone marrow. The bone marrow toxicity has to be considered. Patients previously treated with methotrexate are very sensitive to thioguanine and close follow up is mandatory with adjustment of the thioguanine dose according to blood white cell and thrombocyte levels.