Reduced single-breath transfer factors of the lung for carbon monoxide are seen in a number of conditions. The hypothesis of the present study was that self-reported respiratory symptoms differ in their prediction of TL,CO level in a general population in Norway. A cross-sectional survey of a general population sample in Norway, made up of 1,275 subjects aged 18-73 yrs, was conducted in 1987-1988. The attendees (84% response rate) filled in a questionnaire on respiratory symptoms and underwent standardized spirometric and TL,CO measurements and clinical examination. Associations between TL,CO and respiratory symptoms were assessed using multiple regression models. Tests for interaction were used to examine whether these associations varied with sex, age and smoking habits. Data from 1,221 subjects were analysed. Both males and females who reported respiratory symptoms had significantly lower TL,CO after adjusting for age and height. In a multiple linear regression analysis of TL,CO, adjusting for sex, age, height and smoking habits, the symptoms for which statistical significance was attained, were morning cough, chronic cough and breathlessness. However, only the breathlessness score was independently associated with the TL,CO (-0.42 mmol x min(-1) x kPa(-1) per breathlessness score unit) after adjusting for other respiratory symptoms, and the relationship was stronger in males than in females. In this study, the strongest predictor for a decreased single-breath transfer factor of the lung for carbon monoxide was the presence of self-reported breathlessness, regardless of age, smoking habits and height.