This paper is a report of 100 patients with ascites undiagnosed by the usual diagnostic procedures but definitively diagnosed by peritoneoscopy. The causes of ascites in these patients were, in order of frequency, cirrhosis (34), metastatic malignancy (24), tuberculous peritonitis (23), hepatoma (13), pancreatitis (4), myxedema (1), and constrictive pericarditis (1). Helpful clinical hints are provided for the differential diagnosis of these conditions. The best laboratory test in the diagnosis of ascites is the study of the ascitic fluid. Cirrhosis and constrictive pericarditis had fluids containing less than 3.0 gm of protein per 100 ml, indicating a transudate. Carcinomatosis, hepatoma, tuberculous peritonitis, and pancreatitis had fluids containing more than 3.0 gm of protein per 100 ml, indicating an exudate. A scheme is presented which should help in the differential diagnosis of most cases of ascites.