The clinical course of cystic fibrosis in nine Pakistani Asians was compared with 18 non-Asian age and sex matched controls. The Asian patients grew Pseudomonas aeruginosa at an earlier age (4.0 v 7.5 years), tended to have lower respiratory function test results (forced vital capacity 58.5 v 76.8% predicted; forced expiratory volume in one second 79.8 v 100.3% predicted), and had significantly greater concentrations of immunoglobulin IgG (13.4 v 10.1 g/l). They had a lower weight for age (78.4 v 95.7%) and weight for height (90 v 98.5%) despite similar intakes of dietary energy. Four of the nine Asians carried the delta F508 mutation compared with 17 of 18 controls. All the Asian patients were born in the UK; seven of their mothers were born in Pakistan and had moderate or severe difficulties with the English language. It is concluded that Asian patients may have a more severe clinical course than matched controls and that genetic and environmental factors may be contributory.