This report documents the disturbance in gastrointestinal motor function in a patient with selective cholinergic dysautonomia that occurred following acute infectious mononucleosis. Apart from the gut, other organs affected included the pupils, sweat glands, lacrimal and salivary glands, and urinary bladder. Autonomic function tests showed the preservation of sympathetic adrenergic functions in contrast to the generalized involvement of postganglionic parasympathetic and sympathetic cholinergic nerves, including denervation hypersensitivity of the pupil and urinary bladder to exogenous cholinergic agonists. Cardiac and abdominal vagal responses were abnormal. Colon myenteric plexus ganglion cells were normal by morphological and immunohistochemical studies, suggesting that the selective cholinergic dysautonomia was the most likely pathophysiologic process responsible for the observed motility disorder. This study documents the occurrence of selective cholinergic dysautonomia following a viral illness, the importance of the extrinsic neural control on the motor function of the gastrointestinal tract, and the usefulness of combined motility and autonomic function testing in the evaluation of patients with symptoms suggestive of gut dysmotility.