A group fo 237 patients in a long-term lithium treatment were questioned specifically about five side effects commonly associated with lithium treatment and unspecifically about "other" complaints. About one tenth of the patients did not complain of side effects, two thirds had one or two complaints, and one fourth had three or more. About one half of the patients complained of hand tremor, two thirds of increase thirst, one fifth of weight gain exceeding 10kg, one fifth of diarrhea, and one tenth of edema of legs or face. A few patients had other complaints. For each side effect we analysed whether its presence was significantly associated with such patient and treatment variables as sex, age, duration of the lithium treatment, 12-h serum lithium concentration, type of lithium preparation, and additional medication. Men complained of tremor significantly more often than women. Diarrhea was significantly less frequent in patients given additional treatment with antidepressants. Weight gain was associated with increased thirst and fluid output and with significantly increased blood pressure. None of the other variables distinguished between patients with and without the various complaints. We nevertheless hypothesize that a moderate reduction of the serum lithium level may lead to a lowering of the frequency of some side effects.