Pulmonary function tests are objective evidence of the severity of asthma. A cross-sectional study was carried out to determine the sensitivity of various pulmonary function indices in picking up clinically diagnosed mild and severe asthma. Three groups, each with 60 subjects between 5 and 15 years all of either sex, with mild asthma, severe asthma, and without asthma, respectively, were studied. Pulmonary function tests were performed using a portable spirometer. FEV1 and FVC could differentiate mild asthma from non-asthmatic children in 38 (63 per cent) and 35 (58 per cent), respectively. FEF25% and FEF75% could identify 46 (77 per cent), and 47 (78 per cent) of mild asthmatic children. In children with severe asthma, FEV1, FVC, FEF25%, and FEF75% were abnormal in 54 (90 per cent), 48 (80 per cent), 58 (97 per cent) and 56 (94 per cent), respectively. Peak expiratory flow rate was abnormal in 77 per cent of mild and 87 per cent of severe asthmatics. The FEV1/FVC ratio showed no significant difference between asthmatics and non-asthmatics. It is concluded that FEF25% and FEF75% are better indices for assessment of severity of asthma than FEV1 and FVC. The ratio FEV1/FVC is not useful.