Weight gain achieved during pulmonary tuberculosis (PTB) treatment is associated with the likelihood of bacteriological treatment success. It is recognised that weight and body mass index (BMI) characteristics differ between ethnic groups in health and illness states. However there has been no prior investigation of how ethnic differences in BMI might influence tuberculosis treatment outcome. Our aim was to investigate predictors of microbiological response to PTB treatment at the Tuberculosis Clinic in Timika, Papua Province, Indonesia and specifically, to determine the contribution of ethnicity. The population comprises two distinct ethnic groups - Asian (Non-Papuan) and Melanesian (Papuan). We conducted a prospective study of adults with smear-positive PTB. Treatment outcomes were 1- and 2-month sputum culture and time to microscopy conversion. Clinical measures included weight, BMI, chest radiograph, pulmonary function including forced expiratory volume in 1 second (FEV1) and haemoglobin. One hundred eighty six participants (83 Papuan, 103 non-Papuan Indonesians) were enrolled. At baseline, Papuans had higher mean weight and BMI than non-Papuans (50.0 kg versus 46.9 kg, p = 0.006 and 20.0 kg/m2 versus 18.7 kg/m2, p = 0.001 respectively). This was despite having lower mean haemoglobin (11.3 vs 13.1 g/dL, p<0.0001), higher smoking and HIV rates (37% vs 21%, p = 0.02 and 20% vs 5%, p = 0.01 respectively) and longer median illness duration (3 vs 2 months, p = 0.04), but similar radiological severity (proportion with cavities 55% vs 57%, p = 0.7), sputum smear grade (p = 0.3) and mean % predicted FEV1 (63% vs 64%, p = 0.7). By 2 months, Papuans had gained still more weight (mean 5.9 vs 4.2 kg, p = 0.02), and were more likely to have negative sputum culture (49/56 vs 45/67, p = 0.02), in univariable and multivariable analyses controlling for other likely determinants of culture conversion. In conclusion, Papuans had better early microbiological outcome from PTB treatment, which may relate to better preservation of weight and greater early weight gain.