There are limited data regarding the effects of activin A on underweight, normal weight, and overweight patients with asthma. We determined serum levels of activin A in asthmatic patients in relation to body mass index. The study protocol included questionnaires, measurement of exhaled nitric oxide, blood sampling for inflammatory biomarkers, and high-resolution computed tomography of the lungs to identify bronchial wall thickening. Serum and sputum activin A levels were measured using an enzyme-linked immunosorbent assay in 94 asthmatic patients. Mean serum levels of activin A were significantly (P = 0.001) higher in underweight (1781 ± 327.3 pg/mL) than in normal weight and overweight asthmatic patients, regardless of gender. After stratification by gender, significantly higher mean values of activin A were observed in females compared to males in the normal and underweight groups (P = 0.003 and P = 0.0002, respectively). Significant differences between groups were found in airway wall area (%) (P < 0.0001). We also observed a much higher percentage of sputum lymphocytes in the underweight group compared to the other groups (P < 0.0001). Correlations between bronchial wall thickness and activin A were found in the underweight (r = 0.67, P = 0.48) and normal weight groups (r = 0.51, P = 0.042). Correlations between fractional of exhaled nitric oxide, forced expiratory volume in 1 s, delayed treatment years, and activin A in different groups were also observed. Increased serum level of activin A indicates its role in the pathogenesis of asthma, particularly in underweight and overweight patients.