The EORTC QLQ-C30 Health-Related Quality of Life (HRQOL) questionnaire was completed by 3069/3919 (78%) of a random sample of the Swedish population aged 18-79 years. The aims of the study were to provide normative data on the questionnaire and to investigate differences in HRQOL with respect to age, gender, sociodemographic characteristics and reported chronic health problems. Women had lower scores than men on all but one of the EORTC QLQ-C30 subscales and reported more chronic health problems. The oldest respondents (70-79 years) had a greater degree of impaired HRQOL than the other age groups, with one exception, 'Emotional functioning', in which they scored higher. Unemployed respondents reported poorer HRQOL than employed respondents. Higher income was associated with a more positive assessment of HRQOL. The results of the study present reference values for EORTC QLQ-C30 Version 3 questionnaire and clarify the influence of factors which should be taken into account when planning studies of HRQOL.