In a search for host factors for the complex multifactoral disease, chronic obstructive pulmonary disease, we compared some major components of the complement system in 111 subjects with forced expiratory volumes in 1 sec less than 70% of predicted (index subjects) to those in 111 subjects matched as pairs for age, sex, smoking, and occupation having forced expiratory volumes in 1 sec more than 85% of predicted normal. Significantly lower blood levels of C3 and C4 were found in the index subjects (p less than 0.001). Among the index subjects there was a significant correlation between chronic cough and expectoration and low C3 and C4. There was not a significant correlation between low values and the frequency and severity of past respiratory infections. Levels of serum B globulin fraction, in which C3 and C4 migrate, were not different between the index and control subjects.