Compliance with long-term lithium treatment was studied retrospectively in 73 patients suffering from major affective disorders. It was assessed during the first two years treatment by monitoring plasma lithium levels every 6 weeks. 5 patients stopped the lithium treatment before the first two years were up. Of the 68 remaining patients, only 32 (47%) were thought to be complying perfectly. Most of the failures with lithium (11 patients out of 15) were observed in non-compliant patients. The main factors accounting for non-compliance were a low intellectual level, cognitive deficit, affective relapses and traits of personality disorders; on the other hand, neither the age nor the sex of the patients, nor the polarity, the severity or the duration of the affective disorders, nor the modalities or side effects of the treatment seemed to have any bearing on compliance.