Urinary incontinence (UI) is a common female dysfunction, affecting women in all ages. Experienced physiotherapists and experts on low back pain (LBP) have since long observed and discussed the empirical association between LBP and UI. The aim of this study was to describe the occurrence of UI in women with LBP and to compare this group with a reference group of comparable age, language, culture and parity. The authors of this study had previously collected all original data from the reference group. A validated questionnaire concerning UI was answered by 200 consecutive women with LBP visiting specialised physiotherapy clinics in the area of Stockholm. Inclusion criteria were LBP, female, not pregnant, Swedish speaking, age between 17 and 45 years. Seventy-eight percent of the women with LBP reported UI. In comparison with the reference group, the prevalence of UI and signs of dysfunctional pelvic floor muscles (PFM) were greatly increased (p < 0.001) in the LBP group. Logistic regression analysis showed that the condition LBP and PFM dysfunction i.e. inability to interrupt the urine flow (p < 0.001) were risk factors for UI irrespective of parity. Physiotherapists treating patients with LBP should be aware of possible incontinence problems in this patient group.