A clinical trial, aimed at secondary prevention of low-back pain, was performed in 142 hospital employees reporting at least three annual episodes of this condition. Participants were randomly assigned to one of three groups: a calisthenics program (CAL) for 3 months with biweekly sessions of flexion exercises, a back school program (5 sessions), and a control group. The effectiveness of the two intervention programs was evaluated over a 1-year period. Baseline preintervention data and evaluation at the end of 3 months of intervention and after an additional 6 months were collected. A monthly surveillance for the whole year showed a mean of 4.5 "painful months" in the CAL group versus 7.3 and 7.4 months in the back school and control groups, respectively (P less than 0.0001). The superiority of the CAL group was achieved partly because of the significant increase in trunk forward flexion and to initial increment in abdominal muscle strength. The increased trunk flexion was associated with the rate of participation in the CAL sessions. Further research is needed to answer the question of "intensity versus type of exercise" by comparing different intervention programs, with similar intensity.