Forty-six separate gait studies were analysed for 21 ambulatory patients with Duchenne muscular dystrophy. Three groups were defined on the basis of significant gait variables: early, transitional and late. Disease progression can be predicted with 91 per cent accuracy by three gait variables: cadence, dorsiflexion in swing, and anterior pelvic tilt. The patients in the early group manifested a positive Gower's sign but gait changes were subtle, being principally slightly increased hip flexion in swing, decreased dorsiflexion in swing and reduction in cadence. The force line moved in front of the knee center early in single-limb support. In the transitional stage, anterior pelvic tilt was exaggerated, cadence was further reduced, and foot drop in swing phase was increased. Shoulder sway was noted as a compensation for gluteus medius weakness. The base of support widened. The force line remained behind the hip joint and in front of the knee joint throughout single-limb support. In the late stage, work output increased, cadence continued to drop, shoulder sway increased further, and there was a wider base of support. The force line remained very close to the center of the hip and in front of the knee at all times during single-limb support. The authors' conclusions are: (1) the earliest postural change in gait is increased lumbar lordosis secondary to gluteus maximus weakness, and at this time the quadriceps are relatively competent; (2) quadriceps insufficiency was the key factor in gait deterioration. It appeared in the transitional stage and was characterized by exaggerated anterior pelvic tilt, restricted hip extension in stance phase, equinus posturing, and maintenance of the force line in front of the knee throughout single-limb support. Long-leg bracing is indicated when these signs of quadriceps insufficiency are noted.