To compare the diagnostic performance of maternal blood C-reactive protein, white blood cell count (WBC), and amniotic fluid (AF) WBC in the identification of positive AF culture, histologic and clinical chorioamnionitis, and neonatal morbidity in women with preterm premature rupture of membranes (PROM). Maternal blood was collected for the determination of C-reactive protein and WBC at the time of amniocentesis from 90 women with preterm PROM. Amniotic fluid was cultured for aerobic and anaerobic bacteria as well as mycoplasmas. Amniotic fluid WBC was determined for research purposes. Receiver operating characteristic curve and logistic regression were used for statistical analysis. The prevalence of positive AF culture was 28% (25 of 90). Women with positive AF culture and clinical chorioamnionitis had significantly higher median C-reactive protein, WBC, and AF WBC than did women without these conditions (P < .05), whereas women with histologic chorioamnionitis and significant neonatal morbidity had higher median C-reactive protein and AF WBC, but not WBC, than those without the conditions (P <. 05). An AF WBC of at least 20 cells per mm3 had a greater sensitivity than C-reactive protein (cutoff, 0.7 mg/dL) and WBC (cutoff, 13,000 cells per mm3) in the detection of positive AF culture and histologic chorioamnionitis. Logistic regression analysis indicated that among AF WBC, C-reactive protein, and WBC, AF WBC was the best predictor of positive AF culture (odds ratio [OR] 24.2, 95% confidence interval [CI] 6.0, 97.5, P < .001), histologic (OR 74.0, 95% CI 7.4, 736.3, P < .001) and clinical chorioamnionitis (OR 8.9, 95% CI 0.9, 85.6, P = .057), and neonatal morbidity (OR 4.3, 95% CI 1.1, 16.6, P < .05). Amniotic fluid WBC performs better than C-reactive protein and maternal blood WBC in the diagnosis of positive AF culture, histologic and clinical chorioamnionitis, and neonatal morbidity in women with preterm PROM.