Arthrography with hip aspiration was performed in 143 patients with hip arthroplasties to determine its effectiveness as a technique for diagnosing infection. Thirty-three cases of infection were found. On 26 occasions the germ responsible was isolated in the joint fluid. In six other cases infection was revealed from cytologic or arthrographic findings, or from both. Only once was a diagnosis of infection not arrived at. In this series of patients, except in cases of evolutive inflammatory rheumatism, cytologic examination of the joint fluid was a discriminative factor in diagnosis. Hip aspiration arthrography had a sensitivity of 79 per cent for the diagnosis of infection in arthroplasty with isolation of the germ and a specificity of 100 per cent. The sensitivity of the diagnosis rose to 91% when any one of the following features was observed: leucocytosis of the joint fluid higher than 10,000 elements/mm3; presence of a fistula or of fistulization on arthrography; isolation of the germ in the joint fluid or the rinsing liquid.