Fifty-eight consecutive patients who were admitted to two departments of general surgery with ascites of unknown origin were investigated by measurement of ascitic fluid fibronectin concentration in a prospective study. A fibronectin concentration of 100 ml/l or more was chosen as indicative of malignant disease. In 40 patients with malignant disease, the median fibronectin concentration was 160 mg/l (interquartile range 120-270, range 20-560). In 18 patients with ascites from benign disease the corresponding figures were 40 mg/l (30-70, 20-90) (p less than 0.01). The diagnostic specificity (predictive value of a positive test) was 1.00 (34/34) (95% confidence interval 0.90 to 1.00), while the diagnostic sensitivity (predictive value of a negative test) was 0.75 (18/24) (95% CI 0.53 to 0.90). In this series an ascitic fluid fibronectin concentration of 100 mg/l or more indicated malignant disease, but a lower value did not exclude it. The test should be investigated in combination with another test with high diagnostic sensitivity to optimise its diagnostic value.