A clinical, biochemical, and pathological study is recorded of 50 patients in whom a diagnosis of idiopathic steatorrhea had been made and who had ;flat' jejunal biopsies. It is suggested that there is an underlying constitutional defect, not yet clearly defined, and that possibly secondary intestinal infection allows the intestinal mucosa to become sensitized to substances in the diet. Of these gluten is the commonest; milk adversely affects others; and there may be other factors.