Infection of the intervertebral space is a rare complication of operation on the disc (Ford and Key 1955). It occurred in five cases in a series of 650 operations for lumbar disc protrusion done by two of the authors of this paper--an incidence of 0-8%. Symptoms began between seven and twelve days after operation. After initial relief of the original sciatic pain, the patients developed severe back pain and muscle cramps in the legs. Straight-leg raising became much more limited than before operation. The erythrocyte sedimentation rate was raised in all cases. The diagnosis was confirmed by rescreening the Myodil, when an indentation was seen opposite the intervertebral space from which the disc had been excised. Early removal of infected material from the disc space resulted in complete relief of pain and cure of the infection.