PROBLEM: To assess the usefulness of amniotic fluid (AF) granulocyte colony stimulating factor (G‐CSF) levels as a rapid marker for diagnosing chorioamnionitis. METHOD OF STUDY: AF levels were obtained from term and preterm patients with and without chorioamnionitis (CAM). Patients with urinary tract, respiratory tract, and other infections were excluded. Results obtained from the AF G‐CSF assays were compared with those from other parameters used for diagnosing CAM: maternal fever, leukocytosis, tachycardia, fetal tachycardia, AF glucose levels, white blood cell count, Gram stain, and aerobic and anaerobic cultures. The sensitivity, specificity, and predictive values were calculated. RESULTS: In the uninfected AF samples, G‐CSF levels were present but low, ranging from 400 to 1600 pg/ml. Levels in the infected samples, however, were markedly increased, ranging from 1600 to 14,000 pg/ml; P < 0.05. When a cutoff of 2000 pg/ml was used as a clear marker for CAM, the sensitivity was 67%, the specificity was 100%, and the positive and negative predictive values were 100% and 86%, respectively. The comparison of the other AF G‐CSF laboratory parameters also revealed high sensitivity, specificity, and predictive values for detecting CAM. CONCLUSION: (i) AF G‐CSF levels are elevated in CAM. (ii) An AF G‐CSF level >2000 pg/ml is a strong positive predictor of CAM. (iii) Elevated AF G‐CSF levels appear to be more reliable in predicting CAM than any other single test currently used in clinical practice.