The diagnosis of periprosthetic hip infections is often challenging. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level blood laboratory tests are commonly used to aid in the diagnosis. We studied the sensitivity, specificity, and false-negative rates of ESR and CRP level in a prospective group of patients who underwent revision total hip arthroplasty between 2000 and 2008. Seventy-seven patients with periprosthetic hip infections and ESR and CRP data were identified. Chi-square analysis was performed to determine the significance of false-negatives, compared with sex, body mass index, primary diagnosis, infection type, and immunity status. ESR had 89% sensitivity and 69% specificity. CRP level had 93% sensitivity and 40% specificity. The false-negative rate was 10.8% for ESR and 7% for CRP level. The false-negative rate for ESR and CRP level combined (with either result positive) was 3%. All false-negatives in the combined group were immunocompromised. Chi-square analysis did not find a significant correlation between false-negatives and any other variables. ESR and CRP level are useful in the diagnosis of periprosthetic hip infections. Ordering these tests concurrently reduces the chance of false-negative results.