This study examines the association between somatic co-morbidity and both general and disease-specific health-related quality of life (HRQoL) in patients with asthma and chronic obstructive pulmonary disease (COPD). A cross-sectional analysis was done among 161 COPD patients and 395 asthma patients, aged 40-75 years, recruited from general practice. In the total study population, 47% had no, 32% had one, and 21% had two or more somatic co-morbid conditions, with no significant differences between asthma and COPD patients. Co-morbidity appeared to be associated with poor disease-specific HRQoL in asthma [odds ratio (OR) = 2.08 (1.37-3.18)] and with poor general HRQoL in asthma [OR = 2.96 (1.93-4.53)] and COPD [1.81 (0.91-3.60)] patients. Poorest HRQoL was found in patients with more than one co-morbid condition. Cardiac disease and hypertension were associated with poor disease-specific HRQoL in asthma. Of all co-morbid conditions, musculoskeletal disorders were most strongly associated with poor general HRQoL. Cardiac disease was found to be associated with general and disease-specific HRQoL in asthma but not in COPD. In studies on patients with asthma or COPD aged 40-75 years, co-morbidity should be treated as a determinant of HRQoL.