Objectives: To examine the construct validity of the International Fitness Scale (IFIS) (ie, self-reported fitness) against objectively measured physical fitness in women with fibromyalgia and in healthy women; and to study the test-retest reliability of the IFIS in women with fibromyalgia. Design: Cross-sectional study. Setting: Fibromyalgia patient support groups. Participants: Women with fibromyalgia (n=413) and healthy women (controls) (n=195) for validity purposes and women with fibromyalgia (n=101) for the reliability study. The total sample was N=709. Interventions: Not applicable. Main Outcome Measures: Fitness level was both self-reported (IFIS) and measured using performance-based fitness tests. For the reliability study the INS was completed on 2 occasions, 1 week apart. Results: Women with fibromyalgia who reported average fitness had better measured fitness than those reporting very poor fitness (all P<.001, except 6-minute walk test where P<.05), with similar trends observed in healthy control women. The test-retest reliability of the IFIS, as measured by the average weighted kappa, was .45. Conclusions: The IFIS was able to identify women with fibromyalgia who had very low fitness and distinguish them from those with higher fitness levels. Furthermore, the IFIS was moderately reliable in women with fibromyalgia. (C) 2016 by the American Congress of Rehabilitation Medicine