Ninety painful prosthetic hips from 88 consecutive patients underwent fine needle aspiration and the results compared with clinical progress, operative findings and culture. Fourteen of the 15 cases proven to be infected at operation were correctly identified by fine-needle aspiration. The one hip with a false negative aspirate had an arthrogram which showed a cavity suggesting infection. There were three false positive aspirates. One grew a different organism at surgery from the original aspirate. The other two had no growth on surgical culture. Both had had pre-operative antibiotics. Eleven cultures of doubtful significance were repeated or regarded as negative and the patients carefully followed-up. One repeat culture confirmed infection emphasizing the importance of repeating doubtful aspirates. The sensitivity of aspiration is 93%, specificity is 96%. The accuracy is 95% confirming that aspiration is a simple and reliable method of diagnosing infection in hip prostheses.