The aim of the study was to compare the effect ofwater-based exercise in thermal mineral water versus land-based exercise therapy on the lumbar spine range of motion and physical disability in adult patients with chronic low back pain. Out of 72 patients hospitalized for inpatient treatment in a special rehabilitation hospital, 36 patients performed a 3-week standardized program of group water-based exercises and the other 36 performed a program of group land-based exercises. All patients were also treated with electro analgesic therapy and underwater massage. The patients were assessed for lumbar spine motion using standardized measures with flexible tape, while physical disability was measured by the Physical Disability Index. Evaluations were performed at the beginning and at the end of treatment. Compared with baseline, a statistically significant improvement was detected in both groups regarding both primary outcome measures. At the end of treatment, there was no statistically significant difference between the two exercise treatments in any parameter of interest (p < 0.01). In conclusion, in our sample of patients with chronic low back pain, exercise treatment improved lumbar motion and decreased the level of physical disability. However, comparison of land-based exercises and water-based exercises in thermal mineral water did not demonstrate any significantly different result.