Five hundred children with a painful hip or a limp were evaluated prospectively by plain films and sonography. The clinical, radiographic, and sonographic findings were correlated with the final diagnoses. Sonography disclosed hip effusion in 235 patients, and plain films were abnormal in 58 of these 235 patients and in four others. Both sonography and plain films were normal in 261 patients. No sonographic signs served to differentiate sterile, purulent, or hemorrhagic effusion. Follow-up sonograms were performed in 202 patients. Sonography showed that 73% of patients with presumed transient synovitis had no effusion 2 weeks after diagnosis. Patients with hip disorders other than transient synovitis had persistent effusion for more than 2 weeks; however, that was also observed in 27% of patients with presumed transient synovitis. Sonography was more sensitive than plain films for detecting hip effusion. However, sonographic detection of effusion changed the therapeutic approach in only six patients.