Objective To study the effects of an aquatic therapy program with different frequencies (2 vs 3 days per week) in chronic low back pain. Desing Non-randomized comparison trial. Setting Sport and spa community health club. Subjects Fifty-four adults with chronic low back pain (48.9 +/- 10.0 years). Intervention Eight-week aquatic therapy program. Outcome Measures Pain (visual analog scale [VAS]), disability (Oswestry Disability Index), and quality of life (Short-Form Health Survey 36), body composition (weight, body mass index, body fat mass, body fat percentage, and skeletal muscle mass), and health-related fitness (sit and reach, handgrip strength, curl-up, Rockport 1-mile test). Results Both experimental groups presented significant improvements in low back pain and disability (P < 0.001) compared with control group. The 3 days/week group showed significantly greater benefits at VAS flexion and disability (P < 0.001) than the 2 days/week group. Regarding quality of life, both intervention groups presented significant differences for Physical Role (P < 0.05), Bodily Pain (P < 0.001), General Health (P = 0.012), and Standardized Physical Component (P < 0.001) compared with control group. Both experimental groups significantly improved all health-related fitness parameters (P < 0.01). The 3 days/week group showed significantly greater benefits at curl-up and heart rate (P < 0.001) than the 2 days/week group. No significant changes between treatment groups and control were found in body composition. Conclusions Eight weeks of aquatic therapy program decrease levels of back pain and disability, increase quality of life, and improve health-related fitness in adults with chronic low back pain without effects in body composition. A doseresponse effect was observed in some parameters, with greater benefits when exercising 3 days per week compared with 2 days.