OBJECTIVE: To assess whether inflammation on ultrasound is predictive of clinical response to intraarticular (IA) corticosteroid injections in patients with knee osteoarthritis (OA). METHODS: Patients with symptomatic knee OA were randomized to receive either an IA injection of 40 mg triamcinolone acetonide in the treatment group or 1 cc 0.9% saline in the placebo group. Clinical response was assessed by changes in baseline Western Ontario and McMaster Universities (WOMAC) index scores and physician global assessment at 4 and 12 weeks. Ultrasounds were performed at each visit. Patients and assessors were blinded to treatment status. RESULTS: Seventy-nine patients were enrolled into the study. Four-week data were available for 67 patients in the primary analysis comparing change in WOMAC pain score from baseline to 4 weeks. There was almost no change in the WOMAC pain subscale score from baseline to 4 weeks in the control group, but there was a significant improvement in WOMAC pain subscale score from 10.8 (SD +/- 3.2) at baseline to 8.75 (SD +/- 4.0) at 4 weeks in the treatment group (adjusted p = 0.001). Of the 34 patients in the treatment group; 16 (47%) had inflammatory disease and 18 (53%) had noninflammatory disease as determined by ultrasound. There was no difference in the change in WOMAC pain score between the inflammatory and noninflammatory patients in the treatment group at 4 weeks. There was a statistically significant greater improvement in pain subscale scores among noninflammatory patients than among inflammatory patients at 12 weeks. CONCLUSION: Intraarticular corticosteroid injections are an effective short-term treatment for symptomatic knee OA compared to placebo. Patients with noninflammatory characteristics on ultrasound had a more prolonged benefit from IA corticosteroids compared to inflammatory patients.