The diagnostic usefulness of ferritin measurements in pleural and peritoneal effusions has been evaluated in 57 patients. Mean (+/- standard error [SE]) ferritin levels were 291 +/- 50 ng/ml in 24 patients with noninflammatory transudates (Group I), 942 +/- 253 in 15 patients with nonmalignant exudates (Group II), and 1805 +/- 257 in 18 patients with malignant exudates (Group III). The mean (+/- SE) ratio of effusion/serum ferritin in Groups I, II, and III was 0.7 +/- 0.1, 2.7 +/- 0.7, and 5.7 +/- 1.2, respectively. The specificity and predictive value of a ferritin ratio in excess of 1.5 in distinguishing transudates from all exudates and in distinguishing transudates from malignant exudates were both very high (94%) to 96%). In the lower range of values considerable overlap existed between ferritin ratios obtained in patients with benign versus malignant inflammatory exudates. However, very high ferritin levels (greater than 3000 ng/ml) and ferritin ratios (greater than 20:1) were only encountered in malignant exudates. These results indicate that the measurement of ferritin levels and ferritin ratios may be a useful aid in the diagnosis of malignant pleural and peritoneal effusions.