BACKGROUND: Recent studies suggest that risperidone long-acting injection (RLAI) may be considered for controlling mood episodes in bipolar disorder patients who have relapsed due to medication nonadherence or failure to respond to standard therapies. Currently, no study has reported the usefulness of RLAI in youths with bipolar disorder. The aim of this study was to evaluate short-term effects of RLAI in the naturalistic treatment of early-onset bipolar disorder and its role in symptomatic remission and adherence to treatment. METHOD: Nineteen early-onset bipolar disorder outpatients receiving RLAI were observed in a 6-month naturalistic study at the outpatient clinic of the Child and Adolescent Affective Disorders Program at the Institute of Psychiatry of the University of São Paulo, São Paulo, Brazil. All patients met DSM-IV criteria for bipolar disorder. Clinical response to RLAI was evaluated using the Children's Global Assessment Scale (CGAS) and Clinical Global Impressions scale (CGI) across 3 time periods: index time (T0), 8 weeks after (T1), and 24 weeks after (T2). These subjects were recruited from May 2008 to December 2009. RESULTS: Patients receiving RLAI presented considerable improvement in global functioning (CGAS: T0 = 20.6; T1 = 42.9; and T2 = 49.2) and clinical severity (CGI: T0 = 5.9; T1 = 3.9; and T2 = 3.4). Global CGI mean scores of clinical improvement were 2.2 at T1 and 2.4 at T2. There were no significant changes in laboratory measurements and weight throughout follow-up. CONCLUSIONS: RLAI was shown to be an alternative treatment for youths with bipolar disorder failing to respond to prior medication trials or with adherence problems. Further blind, randomized controlled studies are necessary to confirm these initial findings. TRIAL REGISTRATION: Sistema Nacional de Informaçoes Sobre Ética em Pesquisa Envolvendo Seres Humanos-Commisão Nacional de Ética em Pesquisa identifier: CAAE 0709.0.015.000-06.