A 30-year-old cocaine-dependent man who was a subject in a study evaluating the anticraving efficacy of the stimulant medication diethylpropion (DEP) became manic during his second week on the study drug. Pupillometric changes while on DEP, especially changes in the total power of pupillary oscillation, were dramatically different than those observed in the eight other study subjects who did not become manic. The large changes in total power of pupillary oscillation occurred a few days before the patient became fully manic. Such medication-associated changes in the total power of pupillary oscillation might be of utility in identifying persons at risk for manic-like adverse effects during the medical use of psychomotor stimulants or sympathomimetic agents.