The presence of CA125 was assessed in peritoneal fluid from 70 patients with ovarian cancer and 32 control patients. The follow-up period ranged from 39 to 89 months (median, 56 months). The cutoff for normal peritoneal fluid CA125 levels was determined to be 250 U/ml. A positive correlation between the serum and peritoneal fluid CA125 levels was observed (P less than 0.001). Peritoneal fluid levels were higher than serum levels in all patients. Patients with evidence of active ovarian cancer showed higher peritoneal fluid CA125 levels than the control patients (P less than 0.001). Peritoneal fluid CA125 levels correlated inversely with survival (P = 0.004). The peritoneal fluid CA125 levels were higher in patients with bulky tumor than in those with small (less than 1 cm) tumors (P less than 0.001). Eight out of twenty-six patients with active cancer and available peritoneal cytology had a negative peritoneal cytology. Three of these patients showed elevated peritoneal fluid levels. Three patients out of twenty-four showed elevated peritoneal fluid CA125 levels at second-look laparotomy. These 3 patients had negative biopsies at second-look surgery, but relapsed during the observation period. At second-look laparotomy an elevated peritoneal fluid CA125 level may imply a bad prognosis, but a normal level does not exclude the presence of disease.