N-3 fatty acids have been reported to be effective for asthma. In the present study, the effects of perilla seed oil (n-3 fatty acids) on asthma were compared with the effects of corn oil (n-6 fatty acids) in terms of pulmonary function, lipometabolism and the generation of leukotriene C4 (LTC4) by leucocytes. A total of 28 asthmatic patients were randomly divided into two groups : Group A patients (15 subjects) consumed perilla seed oil-rich supplementation, while Group B patients (13 subjects) consumed corn oil-rich supplementation for 4 weeks. Generation of LTC4 by leucocytes, respiratory function and the serum levels of lipids were compared between the two groups. The generation of LTC4 by leucocytes decreased significantly in Group A subjects following perilla seed oil-rich supplementation for 2(P < 0.05) and 4 weeks(P < 0.01). A significant difference in the generation of LTC4 was observed between the two groups after different dietary supplementations for 4 weeks (P < 0.05). Significantly increased values for PEF (P < 0.01), FVC (P < 0.05) and FEV1.0 (P < 0.05) were found in Group A subjects following perilla seed oil supplementation for 4 weeks, compared with the initial value prior to supplementation. A significant decrease in the serum level of total cholesterol, LDL-cholesterol and phospholipid was detected in Group A subjects following perilla seed oil supplementation for 4 weeks. The present results suggest that peril la seed oil-rich supplementation is effective in the treatment of asthma in terms of its ability to suppress LTC4 generat ion by leucocytes, and in inducing an improvement in pulmonary function associated with changes in the serum level of lipids.