OBJECTIVE: The aim of this study was to examine the use of stimulants among youths in a large and diverse health plan in California from 1996-2000. METHODS: Computerized pharmacy, outpatient visit, and membership files were used to conduct a population-based analysis of stimulant use among over 500,000 enrollees aged 2-18 years. Annual prevalence rates of stimulant use were estimated by calendar year, age, gender, and geographic area. RESULTS: The annual percentage of continuously enrolled 2- through 18-year-olds receiving at least one prescription for a stimulant rose from 1.86% (95% confidence interval (CI) 1.82-1.90) in 1996 to 1.93% (95% CI, 1.90-1.96) in 2000. An increase was observed only among girls 8 years of age or older and among boys 12 years or older. Use of methylphenidate decreased, whereas use of extended-release amphetamine products increased. Stimulant use was lower and use of other psychotropic drugs was higher among children with visits for attention-deficit/hyperactivity disorder (ADHD) and a coexisting psychiatric disorder than among those seen for ADHD alone. Of the 11,698 children receiving at least one stimulant in 2000, 24% of these children received a single prescription. Approximately 55% of stimulant prescriptions were written by physicians in pediatrics and 45% by physicians in psychiatry. CONCLUSIONS: The prevalence of treatment with stimulants was lower than what generally has been reported in other parts of the United States; it increased by approximately 4% between 1996 and 2000, and was frequently short term or intermittent.