Primary adenocarcinoma of the fallopian tube is an uncommon female genital tract tumor. In situ or minimally invasive tubal cancer often poses a diagnostic challenge because of the absence of specific clinical and radiological findings. Here, we report a rare case of bilateral microscopic fallopian tube cancer that initially presented with malignant findings in an endometrial cytologic smear. The patient was a 57-year-old postmenopausal woman. An endometrial smear performed during a routine checkup revealed clusters of malignant cells. During the clinical diagnosis of endometrial cancer, she underwent bilateral salpingo-oophorectomy, total hysterectomy, and partial omentectomy. Histologically, no carcinoma was found in the uterus or ovaries. In toto sectioning of the fallopian tubes and their fimbriated ends revealed minute foci of serous adenocarcinoma in the left tubal mucosa and right fimbria. In situ adenocarcinoma components were present in both lesions. Microinvasive carcinoma was observed in the right fimbria. Although there was no macroscopic dissemination in the peritoneum at the time of surgery, positive intraoperative peritoneal cytology suggests that the patient was at risk of developing peritoneal serous carcinoma if not treated. Clinicians and pathologists should consider the possibility of early tubal cancer when there is no clinically detectable pelvic mass, and malignant cells appear in endometrial cytology specimens.