Data are presented from a case series of 22 patients who abused methylphenidate hydrochloride (Ritalin-SR). The abuse pattern and symptoms of toxicity were similar to that seen with cocaine hydrochloride and amphetamine sulfate addiction; yet, the morbidity and mortality seen in this case series were greater than usual for a group of patients involved in intravenous drug abuse. We describe the characteristics of the methylphenidate abuse syndrome in terms of the pharmacology of methylphenidate, the constituents of the Ritalin-SR preparation, and the disease of chemical dependence. We propose solutions to the problem of methylphenidate abuse.