The associations between four major blood groups, ABH secretor status, and complement C3, and chronic bronchitis and peak expiratory flow were examined in 3387 men, aged 55-74 years. Presence of chronic bronchitis was assessed using the British Medical Research Council (BMRC) questionnaire. Men with NS- in the MNS system had significantly less chronic bronchitis than others, i.e. 11.4% versus 16.0% (p < 0.0005). In heavy smokers, both men with NS- and NS+ had significantly less chronic bronchitis than others. Also, Rhesus positive men had less chronic bronchitis than Rhesus negative men, 28.0% versus 38.3% (p < 0.05). Absence of the complement C3 allele C3F was associated with a higher prevalence of chronic bronchitis. No association with chronic bronchitis was found for the AB0 system or the ABH secretor system. No convincing associations were found between genetic markers and mean peak flow. In conclusion, this study has suggested new genetic markers for respiratory disease indicating a partly genetic aetiology of chronic respiratory disease. The MNS, and to some extent the Rhesus blood groups, were associated with chronic bronchitis in middle-aged men. However, this study demonstrates once again that no single blood type can act as a risk indicator of chronic respiratory disease.