To assess the involvement of complements (C3, C4) in the pathophysiology of bronchial asthma.Selection of patients (n = 64) were made according to the recommended international criteria for diagnosis and classification of asthma. Serum levels of complement components (C3, C4) were measured by radial immunodiffusion technique in 64 Libyan children (age: 1-12 years, sex: 39 males, 25 females) with mild to moderately severe asthma (Group A). Among these patients, 35 had active disease (AA) and 29 had inactive disease (NA). According to age range, 20, 21 and 23 patients were between 1-3 years (A1), > 3-5 years (A2) and > 5-12 years (A3) respectively. A1 had 9 and 11 patients with active (AA1) and inactive (NA1) disease; A2 had 10 and 11 patients with active (AA2) and inactive (NA2) disease; A3 had 16 and 7 patients with active (AA3) and inactive (NA3) disease respectively. Age matched comparisons were made with 57 healthy children (age: 1-12 years; sex: 30 males, 27 females) (Group B). Among the controls, 15, 19 and 23 children were between 1-3 years (B1), > 3-5 years (B2) and > 5-12 years (B3) respectively.Mean C3 level was significantly elevated in patients, while C4 level was normal (A vs B --> C3: P 0.2). Serum C3 level was significantly higher in patients with active disease only, while it was normal in patients with inactive disease (AA, NA, B --> P = 0.045); AA vs NA --> P P P > 0.05) and C4 levels were normal in both the groups (AA, NA, B --> P = 0.354). Further, C3 levels were significantly elevated in all the age groups, but in patients with active disease only (AA1, NA1, B1 --> P = 0.0024; AA2, NA2, B2 --> P = 0.0411; AA3, NA3, B3 --> P = 0.0102).The elevated C3 level was possibly due to induction by pro-inflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1). The probable mechanisms of C3 involvement in the pathophysiology of bronchial asthma were discussed.