OBJECTIVE: To examine persistence with methylphenidate (MPH) therapy among children and youth in the general population. METHOD: We conducted a retrospective analysis of longitudinally organized, individual-specific anonymous data from linked prescription and health databases covering the population of British Columbia for 1990 through 1996. No prescriptions being filled for 4 months indicated cessation of one bout of therapy. RESULTS: Among 16,945 identified MPH patients aged 19 years or under, overall duration of therapy was 584 days, and the average number of prescriptions received was 6.6. One-third of patients received 2 or fewer prescriptions, while 18% followed a chronic, continuous course. Among patients receiving more than 2 prescriptions, 50% of discrete therapy bouts lasted 4 months or less, and one-third of cases had multiple bouts of therapy (range 2 to 6 bouts). Younger age (that is, children aged 0 to 8 years, vs those aged 10 to 19 years), male sex, and receipt of initial prescription from a psychiatrist were associated with greater persistence. CONCLUSIONS: Enormous variability in persistence with MPH therapy and often-occurring low rates of persistence raises questions about the diligence with which MPH patients are selected, prepared, and followed in the general population. Special attention to the needs of older children and to the needs of girls is required, and discontinuities during children's therapy courses require explication.