The authors conducted an 18-month naturalistic prospective follow-up study of 37 adolescents whose bipolar I illness had been stabilized with lithium carbonate during inpatient hospitalization. Thirteen of the patients discontinued prophylactic lithium therapy shortly after discharge. The relapse rate of bipolar illness in these 13 patients was nearly three times higher than the rate in patients who continued lithium prophylaxis without interruption. Early relapse among lithium-treated patients was associated with a greater risk of relapsing again. The authors discuss the theoretical and clinical implications of these findings.