Discitis is a rare complication of disc operation. The incidence rate varies from 0.2 to 0.8% according to the series. During a 6 year period (1980-1986) 1,796 patients were operated for lumbar disc protrusion at our institution and twelve of them (0.66%) developed a post operative discitis. Bacteriologic verification due to the infection was ascertained in ten cases. Direct contamination during surgical time is likely far more frequent than hematogenous contamination because the liable germ was staphylococcus in 9 cases. Ascertaining the diagnosis is base upon clinical picture and some selected investigations. It may be earlier than it has been said before. Discitis may be suspected within a week after operation in two cases out of three. The most prominent clinical feature is back pain with muscle spasm but sometimes diagnosis may be misled to a psychiatric condition or a visceral disorder. Among conclusive investigations we range in the first place the needle aspiration of the disc which permitted to isolate a germ nine times out of eleven. Next the bone scan with H.M.D.P. Te 99 (8 Mbq/kg) which revealed a significant uptake pattern in eight cases out of eight. Finally the blood culture which grew five times out of ten. Risks of discitis, i.e septicemia, polysegmental infection or death justify in our opinion an appropriate antibiotherapy during at least 8 weeks. Moreover, in our experience, it is the best antalgic treatment that we can offer and back pain decreases as soon as the second day with antibiotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)