The purpose of this study was to investigate test-retest reliability and concurrent validity of a Swedish version of the Roland and Morris Disability Questionnaire (RM-Sw), and to describe demographic factors in patients with low back pain of at least 4 weeks' duration seeking outpatient physical therapy treatment in primary care settings. Seventy-two patients participated in the study. The intraclass correlation coefficient for a one-week test-retest interval was 0.88. There was moderate positive correlation with measures of perceived disability (r = 0.64, p < 0.001; r = 0.69, p < 0.001) and pain severity (r = 0.54, p < 0.001), and low negative correlation with measures of perceived life control (r = -0.32, p < 0.01) and general activity (r = -0.27, p < 0.05). Gender, education and occupation were only moderately related to RM-Sw scores, explaining 14% of the variance in the scores. It is concluded that RM-Sw is a reliable and valid measure of functional ability in low back pain.