Background Asthma is a common disabling condition, with known environmental and familial risk factors and with their assumed interactions. We wanted to carry out a family study on asthma to address gene–environment interactions at a population level.
Methods We conducted a nation-wide study on familial risks for asthma. An asthma database was constructed by linking the Multigeneration Register on 0–72-year-old subjects to the Hospital Discharge Register for diagnosed asthma cases in Sweden from years 1987 to 2004. Standardized incidence ratios (SIRs) were calculated for affected singleton siblings, twins and spouses by comparing with those whose siblings or spouses had no asthma.
Results A total of 67 370 hospitalized cases and 5638 affected siblings were identified with a familial SIR of 3.70, which was independent of sex, diagnostic age and age differences between the siblings. The SIR for different-sex twins was 5.17 and for same-sex twins it was 13.38. There was a low correlation between spouses for asthma.
Conclusions Asthma shows a higher familial risk than many common diseases. The higher difference in familial excess risk between singleton siblings and different-sex twins provides strong genetic epidemiological evidence for gene–environment interactions in asthma. The concept of gene–environment interactions needs to be accommodated in future aetiological studies on asthma. Data on environmental factors and family history are important for clinical risk estimation.