The purpose of this research was to study the occurrence of extradyadic sexual relationships among Norwegians. We examined what background factors were associated with this type of relationship, and to what extent condoms were used. In 2002, a random sample of 9,852 Norwegians between the ages of 18 and 49 were asked to participate in an anonymous self-administered questionnaire survey on sexual behavior. A total of 3387 persons responded (1386 men, 1993 women, and 8 who did not indicate their gender), yielding a response rate of 34%. Of the 2807 participants who had ever been married or cohabited, 29% of the men and 23% of the women reported experience with extradyadic sexual relationships at some point during their lives. Sixteen percent of the men and 11% of the women reported extradyadic sex during their current marriage/cohabiting relationship. Women were more emotionally attached to the extradyadic partner than men. Factors that predicted occurrence of extradyadic sexual relationships were age, number of years married/cohabiting, number of sex partners, and sexual orientation. In addition, there was a correlation between low self-esteem and extradyadic sexual experience. About 50% of the participants reported no use of contraception during the most recent intercourse with an extradyadic partner. In conclusion, men's extradyadic partner seems most often to be a casual partner, whereas women prefer someone they already know. The choice of extradyadic partner is most likely associated with use of STI protection.