In a study of 25 consecutive patients presenting with ascites, the use of the ascitic/blood glucose ratio in assisting in the diagnosis of tuberculous peritonitis was investigated. In all 13 patients with histologically confirmed tuberculous peritonitis the ratio was less than 0.96, and in 10 patients with non-tuberculous ascites the ratio was greater than 0.96, this difference being statistically significant (P less than 0.001). In 2 patients with probable non-tuberculous ascites but with pulmonary tuberculosis the results were excluded from statistical analysis, as they had received anti-tuberculosis therapy in addition to their other treatment. The conclusion drawn from this study is that the ascitic/blood glucose ratio is a useful test in the differentiation of tuberculous peritonitis from ascites due to other causes.