Twenty-five patients with rosacea were compared with twenty-five control subjects for previous medical history and tests of immune function. Rosacea patients were found to have a higher incidence of disorders of the auto-immune type and were significantly more difficult to sensitize to DNCB than the controls. In addition, twelve of the rosacea patients and eleven other rosacea patients had biopsies which were examined by the direct immunonuorescence technique. In only five was the test negative. In the remainder deposits of IgM and/or IgG and/or complement were found at the dermo-epidermal junction and/or in the dermal collagen. Serum from the rosacea patients was also examined by the indirect technique and in six cases a circulating antinuclear antibody of IgM type was found. It is suggested that altered immune function plays a significant role in the pathogenesis of the disease.