The purpose of the paper is to describe and analyse the prevalence of parallel sexual relationships among adult Norwegians in terms of social background variables, possibility factors and motivational factors. Data stem from two cross-sectional, postal questionnaire surveys in 1987 and 1992 on two separate representative samples of 10,000 Norwegians aged between 18 and 60. The results showed that 16% of the respondents reported one or more parallel sexual relationships. The prevalence of parallel sexual relationships increased with possibility factors, such as the number of travelling days and population density. Among motivational factors, parallel sexual relationships increased with the number of years married/ cohabiting, except for those married the longest. It decreased with increasing age of ®rst intercourse, and increased with the number of partners before the last partnership. Of those not satis®ed with coital frequency with their permanent partner, 22% had parallel sexual relationship experience compared to 12% among those who were satis®ed. Furthermore, more men than women reported parallel sexual relationships; respondents with higher levels of education more often reported parallel sexual activity than those with lower level of education; and subjects born between 1937 and 1956 reported more parallel sexual activity than the other cohorts. When controlled for the number of years married/cohabiting, the predicted trend suggests that for the cohorts born up to 1970, men and women with lower and higher levels of education have become more similar as regards parallel sexual behaviour.