OBJECTIVE: The authors explored the relationship of adherence to treatment with mood stabilizers (lithium, carbamazepine, and sodium valproate) and plasma levels of these drugs to future psychiatric hospitalizations. METHOD: They prospectively followed 98 patients with mood disorders who were prescribed mood stabilizers. These patients participated in an initial interview and completed a questionnaire regarding their adherence to the medications. Data on their plasma levels of these drugs were taken from assays done in the 3 months before the interview. RESULTS: Six of the 98 patients were not classified as to medication adherence or plasma levels. Twenty-nine (32%) of the remaining patients reported partial adherence to the medication regimen, and 33 (36%) had an index plasma level that was suboptimal. At 18 months, rates of admission to a psychiatric hospital were significantly higher in the 16 partially adherent patients with subtherapeutic plasma levels (N=13, 81%) than in the 46 adherent patients with therapeutic plasma levels (N=4, 9%). However, hospital admission was also more likely in partially adherent patients with therapeutic plasma levels than in adherent patients with subtherapeutic plasma levels. CONCLUSIONS: Adherence to medication regimens may be a proxy measure of other healthy behaviors.