It has been suggested that asthma increases the risk of lung cancer in males but not in females. However, previous studies may suffer from report bias and are based on a small numbers of cases. The objective of the present study was to assess the incidence of lung cancer in males and females using a nationwide Swedish cohort of asthma patients. Patients (n=92,986) aged > or = 20 yrs with a hospital-discharge diagnosis of asthma and who were alive and free from malignancy I yr after first hospitalization were followed-up, for incidence of lung cancer during the period 1965-1994 (average duration of follow-up, 8.5 yrs). Their incidence of lung cancer was compared with that of the national population. The authors observed 713 lung cancers (standardized incidence ratio (SIR) 1.58, 95% confidence interval (CI) 1.47-1.70). The SIR was 1.51 in males (95% CI 1.38-1.65, 492 cases) and 1.78 in females (95% CI 1.55-2.03, 221 cases). The SIR decreased with duration of follow-up and increased with calendar period and age at first hospitalization. The risk of lung cancer was higher for squamous cell and small cell carcinoma than for adenocarcinoma, and it was higher in patients with other diseases as the main diagnosis and in patients hospitalized in departments other than internal and respiratory medicine. It was confirmed that asthma patients are at increased risk of lung cancer, but there is no heterogeneity in risk between the sexes. Several indirect arguments point towards a noncausal explanation of these findings; in particular, confounding by tobacco smoking is a plausible explanation.