Seven patients with severe psoriasis were treated with the aspartate carbamylase inhibitor N-phosphonacetyl-L-aspartate (PALA). Three patients showed a definite improvement lasting up to 3 months after each course of therapy. Three other patients showed more limited responses of shorter duration, and one patient experienced progressive worsening of his psoriasis while on the study. Psoriatic arthritis did not appear to be improved by the PALA therapy. Side effects were usually mild and limited to skin irritation and diarrhea. The development of alternate schedules of administration may lead to a more useful treatment response in patients with severe psoriasis.