In order to expose malignant cells to high concentrations of cytotoxic chemotherapeutic agents, drugs can be instilled directly into the peritoneal cavity of patients with ovarian carcinomas through a Tenckhoff catheter. The peritoneal dialysate removed during such therapy can be examined cytologically for the presence of carcinoma cells. The cytologic specimens from Tenckhoff catheters from 40 consecutive patients with primary ovarian (39) and tubal (1) cancer who received intraperitoneal chemotherapy have been reviewed retrospectively. A total of 237 specimens yielded 78 (33%) positive, 138 (58%) negative, and 21 (9%) inconclusive or suspicious fluids. The major diagnostic problem was the marked mesothelial atypia, which may be related to the high concentrations of cytotoxic agents intimately in contact with the peritoneum. Of the 15 patients who had tissue examined after placement of the catheter (mean interval, 5 months), results agreed with those of the catheter cytologic specimens in ten patients. The catheter cytologic specimen was never positive when histology was negative. Of the 36 patients with evaluable follow-up (mean, 19 months), agreement between the clinical course and the catheter cytologic results was found in 27 patients (75%). Again, interpretation of the catheter specimens was never positive in the face of a benign clinical course. Thus, evaluation of catheter specimens by cytologic examination has a diagnostic sensitivity and specificity of 59% and 100%, respectively. Tenckhoff catheter cytology has proven to be a rather valuable tool to monitor persistent or recurrent intraperitoneal ovarian carcinoma.