This report examines the relationship of long-term medication compliance (as measured by stability of level/dose ratios and total blood level averages) to prophylaxis in recurrent unipolar patients. Of the 53 patients assigned to active imipramine conditions in a long-term maintenance treatment protocol, 42 were evaluated as clearly compliant, 8 were clearly noncompliant, and 3 were not classifiable. We examined a number of demographic and clinical variables in an attempt to find possible predictors of compliance. No differences were observed between compliant and noncompliant patients with respect to sex, age at screening, age at the onset of their recurrent unipolar illness, duration of the index episode, number of previous episodes, bipolar II vs. recurrent unipolar subtype, endogenous vs. nonendogenous subtype, or baseline severity of depression as assessed by the Hamilton Rating Scale for Depression. However, medication compliance was found to be significantly associated with effective prophylaxis. There was a significantly larger proportion (6 of 12) of noncompliant patients among those experiencing a recurrence in one of the two active medication conditions than among those who remained well for 3 years (2 of 28). The value associated with this difference was p = .04. We conclude that effective prophylaxis is strongly related to stability of medication blood levels which fall within the therapeutic range.