Application of dual-time-point FDG PET for mucinous adenocarcinoma has been rarely reported. Isolated perineal recurrence (after neoadjuvant chemoradiotherapy and abdominoperineal resection) was detected by dual-time-point FDG PET in a 41-year-old man with mucinous adenocarcinoma of the rectum. Correlative contrast-enhanced CT demonstrated circumscribed lobulated mass in the fatty tissue of the right perineal region, which was metastatic adenocarcinoma, on fine needle aspiration cytology. An SUVmax increase with time is consistently observed in mucinous tumors.