The data from ten patients who underwent incision and drainage of an acute pyogenic abscess were studied with respect to the presence of an associated hip flexion deformity prior to incision and drainage of the abscess, bacteria cultured intraoperatively, treatment of any persistent hip flexion deformity postoperatively, and any additional sequelae noted at an average follow-up of seven years (range, one to 17 years). Six of the ten patients presented with an associated hip flexion deformity. In three patients, the deformity had resolved spontaneously and completely--in one by the third postoperative day and in two by the 45th postoperative day. Follow-up of these patients at up to 17 years revealed no residual flexion deformity. The flexion deformity in two of the six patients improved following incision and drainage, but residual flexion deformities of 10 degrees and 15 degrees were noted at three- and six-year follow-up, respectively. The sixth patient was treated with skin traction both preoperatively and postoperatively, but it was ineffective in totally correcting the deformity. Follow-up of this patient at one year revealed a residual 15 degrees hip flexion deformity. All six patients had normal ambulation at follow-up examination. Bacteria cultured intraoperatively did not appear to affect either the initial development of the flexion deformity or the time to resolution following incision and drainage of the abscess. Of 183 patients with an acute pyogenic psoas abscess reviewed in 14 series from the literature, 96% (176/183) presented with an associated hip flexion deformity. A hip flexion deformity in a patient with fever and pain on attempted extension of the thigh is therefore a reliable sign for the diagnosis of an acute pyogenic psoas abscess. Treatment may be indicated for a persistent deformity following incision and drainage. Skin traction is the most commonly used and successful method of treatment. Persistence of the hip flexion deformity postoperatively may be secondary to fibrosis within the psoas sheath or actual substance of the muscle, or both. A mild residual flexion deformity may be present years after incision and drainage but apparently will cause no functional abnormality.