Use of Internet versions of questionnaires may have several advantages in clinical and epidemiological research, but we know little about if Internet versions differ with respect to validity and reliability. We aimed to compare Internet- and pen-and-paper versions of short form-36 (SF-36) with respect to test-retest reliability and internal consistency. Women referred to mammography (n = 782) were randomised to receive either a paper version with a prepaid return envelope or a guideline on how to fill in the Internet version. A subgroup was asked to answer the questionnaire once again in the alternative version. Test-retest reliability was assessed by the intra-class correlation coefficient. Internal consistency was calculated as Cronbach's alpha. The between-version test-retest reliability for the eight subscales were between 0.63 and 0.92. Cronbach's alpha for the two versions were all between 0.75 and 0.93 with minor differences between the Internet- and the pen-and-paper version. We found little or no evidence of a difference in test-retest reliability and internal consistency when we compared an Internet- and a pen-and-paper version of SF-36.