Refsum's disease is characterized by the accumulation of phytanic acid (PA). Some clinical features are directly related to serum PA and may therefore improve with plasma exchange (PE). We present a patient with severe polyneuropathy and PA of 1809 mg/L, exacerbated by weight loss of 15 kg. Clinical improvement occurred only when PA fell to 500 mg/L after intensive PE and a high calorie liquid diet. This accords with published reports of severely ill patients which indicate that PE is only effective when combined with dietary control and arrest of weight loss. PE is of questionable value in less severely ill patients with PA < 500 mg/L.