Although well described in the orthopaedic literature and some orthopaedic textbooks, postoperative discitis is regularly missed or diagnosed late. Six cases of discitis were studied in detail with special reference to the clinical presentation. All patients with discitis had an erythrocyte sedimentation rate (ESR) greater than 50 at 2 or more weeks after surgery. A prospective study of 26 patients undergoing uncomplicated discectomy or fusion was made. ESRs were measured preoperatively and at 1,2, and 6 weeks after operation. Any patient with increasing back pain and an ESR greater than 52 or more weeks after surgery should be considered to have discitis until proven otherwise. If the ESR is measured routinely preoperatively and at 2 weeks postoperatively, this condition should not be missed.