Objective: The objective of this study was to describe the prevalence and correlates of adherence to divalproex sodium (DVPX) and lithium carbonate (Li) combination treatment during the initial stabilization treatment phase. Method: Adherence to Li/DVPX combination therapy was measured by the presence or absence of minimum serum concentrations of DVPX (50 micrograms/mL) or Li (0.6 mmol/L). Secondary measures included pill count, patient/parent report, and clinical judgment. Correlates of adherence, including patient characteristics, medication side effects, and family variables, were evaluated. Results: One hundred seven patients (70 males and 37 females) were studied. The proportion of serum concentrations in the therapeutic range across the study period was 0.84 for DVPX and 0.66 for Li. Maternal (r = -0.31; p less than 0.01) and paternal (r = -0.44; p less than 0.01) hospitalization for a psychiatric disorder and less adaptive family functioning (r= -0.26; p less than 0.05) related to treatment nonadherence for DVPX. Better treatment adherence to DVPX (r = 0.21; p less than 0.05) and Li (r = 0.23; p less than 0.05) was associated with a greater number of side effects, whereas male sex was associated with worse adherence to both DVPX (r= -0.24; p less than 0.05) and Li (r = -0.22; p less than 0.05) pharmacotherapy. Clinical response to treatment correlated with adherence to DVPX treatment (r = 0.33; p less than 0.01). Conclusions: Nonadherence may limit the statistical power of treatment efficacy studies and the effectiveness of pharmacotherapy treatment for juvenile BPD and necessitate strategies to evaluate and enhance levels of treatment adherence. (Contains 5 tables.)