Fibromyalgia (FM), pre-dominantly found in women, may accompany other pre-existing rheumatic diseases. The association between FM and ankylosing spondylitis (AS) is uncertain. We evaluated FM in women with AS. Eighteen women with AS were compared with 18 men with AS (controls) for age, duration of symptoms, time to diagnosis, degree of sacroiliac involvement, history of peripheral arthritis, patient global assessment, Health Assessment Questionnaire, Fibromyalgia Impact Questionnaire, level of diffuse pain, Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and Bath Ankylosing Spondylitis Functional Index (BASFI). Physical examination included the number of tender points and enthesitis sites, Schober test, distance between occiput and wall, chest expansion, lateral spinal flexion, and intermalleolar distance. Inflammatory activity was measured by the erythrocyte sedimentation rate (ESR). Of all tested parameters, the ones with significant differences between the groups were time between symptom onset and AS diagnosis (longer for women), FM incidence and the number of tender points and enthesitis sites (higher for women), BASDAI (higher in women and correlated with FM and the number of tender points but not with ESR), and BASFI and BASDAI scores (increased in FM patients). FM was present in 50% of women with AS and associated with higher disease activity indices (BASDAI and BASFI) and not related to severity of physical findings or ESR. The reliability of well-accepted assessment tools of AS, such as BASDAI and BASFI, in evaluating AS activity in women may be called into question due to a confounding effect of FM.