Ross' syndrome is a rare peripheral nervous system disorder defined by Adie's tonic pupil, hyporeflexia, and segmental anhidrosis. Injury to postganglionic cholinergic fibers is believed to account for the tonic pupil and sweating disturbance. We report a 47-year-old man found to have Ross' syndrome in combination with a complete postganglionic Horner's syndrome. Pharmacologic and sudomotor tests in this unique patient provide further evidence that Ross' syndrome results from injury to sympathetic and parasympathetic ganglion cells or to their postganglionic projections.