There are conflicting reports about the benefits of zinc supplements in childhood asthma. This study examined the effect of zinc supplementation in children with asthma attending an outpatient clinic in Sari, Islamic Republic of Iran. In a randomized, double-blind, placebo-controlled clinical trial over 8 weeks, 284 children on inhaled steroids were allocated to receive zinc supplements (50 mg/day) (n = 144) or placebo (n = 140). Cases and controls had low initial serum zinc concentrations [61.8 (SD 7.3) ug/dL and 60.9 (SD 4.3) ug/dLj. After treatment, mean serum zinc level in the case group was significantly higher [129 (SD 20.4) ug/dL] than in the controls [63 (SD 8.6) ug/dLj. There were no significant differences in IgE levels before and after treatment. The case group showed significant improvements in clinical symptoms such as cough, wheezing and dyspnoea and in all spirometry parameters (FVC, FEV1 and FEV1/FVC).