Serial assay of serum CA 125 was carried out in five patients with advanced primary carcinoma of the fallopian tube. Pretreatmental levels of these patients were elevated, ranging from 145 to 535 U/ml (305 +/- 140.1, mean +/- SD). All patients showed rapid decreases in response to treatment; however, reelevations were noted in two patients concomitant with recurrence. In the remaining three patients, who are alive with no evidence of recurrence at 52, 29, and 21 months, serum CA 125 levels remained below 5 U/ml after remission. These results suggest that CA 125 is useful in monitoring patients with primary carcinoma of the fallopian tube.