Two cases are reported exemplifying the difficulties faced in the clinical diagnosis of peritoneal tuberculosis. Two fertile-aged nulliparous females were admitted with symptoms and signs of an acute abdomen. Both showed a relative intestinal obstruction, abdominal mass and ascitic fluid. A malignant disease was suspected and laparotomy was performed. Tuberculous peritonitis was demonstrated histologically in biopsy and later confirmed by positive culture for tubercle bacilli. In the first case the correct diagnosis was disclosed during operation by frozen section, although the histological picture also indicated possible carcinosis because of a heavy mesothelial hyperplasia. A 9-month chemotherapy with isoniazide and rifampicin, supplemented during the first 2 months by streptomycin or ethambutol, was successful in both cases.