SETTING: Tuberculosis control programmes of two health care centres in the central rainforest of Peru. OBJECTIVE: To evaluate if bodyweight gain (BWG) predicts treatment outcome in patients with pulmonary tuberculosis (PTB). DESIGN: Retrospective cohort study of adults with PTB diagnosed between 1995 and 2004. BWG was assessed after month 1 of treatment, after the initial phase and at the end of treatment. Patients were stratified into two BWG categories, ≤5% and >5%. Failures and relapses were grouped together as unsuccessful treatment outcome. RESULTS: A total of 650 patients were included: 7.2% (n = 47) had an unsuccessful outcome. Unsuccessful outcome was associated with BWG ≤ 5% at the end of treatment (RR 2.05, 95%CI 1.10-3.80), but not at the completion of month 1 (RR 0.99,95%CI 0.52-1.88) or at completion of the initial phase (RR 1.46, 95% CI 0.82-2.57). Median BWG at completion of the initial phase was higher in cured patients (P = 0.007). BWG ≤ 5% at end of treatment (RR 2.35, 95%CI 1.17-4.72), initial sputum smear 2+ (RR 2.48, 95%CI 1.14-5.31) and positive smear microscopy at month 2 (RR 4.0, 95%CI 1.30-12.31) were independent predictors of unsuccessful treatment outcome. CONCLUSION: BWG ≤ 5% at the end of treatment, high bacterial load and lack of sputum conversion correlate with unsuccessful treatment outcome in this setting. New discriminative cut-offs for BWG are proposed. © 2008 The Union.