Cancer risk in asthmatic subjects selected from hospital discharge registry.

We wanted to study cancer risk in asthmatic subjects. With the use of central health registries, a cohort of 64,346 patients, treated in hospital with an asthma diagnosis, was followed with respect to cancer development. The observed numbers of different types of cancer were compared with the expected numbers, estimated from population data, with consideration taken to patient age, sex, survival, and the year of diagnosis. In general, a marked reduction of cancer incidence (2 out of 3 of the expected numbers) was found, with the exception of two cancer types: cancer of the respiratory tract and cancer of endocrine glands. A more noticeable reduction in cancer risk was seen for multiple myeloma, malignant melanoma, mammary cancer, uterine body cancer, and stomach cancer. The causes of this "protective effect" are not indicated by the present analysis, and need further study.