The internet is becoming a favored technology for carrying out survey research, and particularly sexual health research. However, its utility is limited by unresolved sampling questions such as how biased internet samples may be. This paper addresses this issue through comparison of a 'gold standard' random selection population-based sexual survey (The Swedish Sexual Life Survey) with an internet-based survey in Sweden which used identical demographic, sexual and relationship questions, to ascertain the biases and degree of comparability between the recruitment methods. On the internet questionnaire, there were significant differences between males and females on all the measured indices. There were no significant differences in proportions of males and females, or nationality, between the two samples. However, the internet samples for both males and females were significantly more likely to be younger, originally from and currently living in a major city, better educated, and more likely to be students and less likely to be retired. Relationship variables were less likely to be significantly different between samples: there were no differences for males or females between the SSS and the internet samples on having been in a committed relationship, and how they met their present partner, nor for males in having discussed separation in the past year. However, there was a higher proportion of people attracted to the same sex, and higher numbers of sex partners (as well as a higher proportion of people reporting no sex) in the past year, in the internet sample. These data suggest that apart from the demographics of age, location, and education, currently being in a committed relationship, and the number of sex partners in the past year, internet samples are comparable for relationship characteristics and history with a national sexual life survey. Comparison of internet data with random survey data in other western countries should occur to determine if these patterns are replicated.