The general mortality rate in Oslo (capital city of Norway) is high compared with the rest of the country, and varies considerably from area to area within the city. This study highlights the differences in cardiovascular risk factors among 40 year-olds in different areas of Oslo between 1985 and 88. A total of 14,220 persons were included in the study. The city was divided into four geographical areas with distinct differences in socio-economic conditions. For both sexes all risk factors were shown to be more favourable in the more affluent areas than in the poorer area of the inner city. Also for both sexes the proportion of persons who smoked on a daily basis varied from 29% in the "best" area to 49% in the "worst" area of the city. The differences in serum cholesterol in the different areas were less pronounced than for the other risk factors. The greatest differences in risk factors were observed among women. The proportion of physically inactive women was 27% in the poorer area compared with 17% in the more affluent part of the city, and the body mass index was above 30 for 8.8% and 2.8% of women respectively. Similarly, the proportion of women with serum cholesterol > 7 mmol/l, triglycerides > or = 2.5 mmol/l, systolic blood pressure > or = 140 mm or diastolic blood pressure > or = 90 mm was almost twice as high for those living in poor socioeconomic conditions than those who were in a better position.