The purpose of this prospective study of 133 affective disorder patients, consecutively referred to start long-term lithium therapy, was to identify predictors of importance for nonadherence. The nonadherent group was defined as the 31 (23.3%) patients who stopped treatment within the first six months. In the multivariate logistic analysis the most informative clinical, social, and psychosocial predictors were, in rank order: many admissions to mental hospitals, death or divorce of parent in childhood, heavy smoking, short duration of the mental disorder diagnosed as affective, not married, never economically active, and early onset of the affective disorder. The logical consequence of the results is to offer the subgroup of patients with an individually calculated high probability of nonadherence intensive control and support during the first month of lithium treatment.