The gross and routine microscopic features of 25 stage I primary mucinous ovarian carcinomas without clinical evidence of recurrence and 25 mucinous carcinomas metastatic to the ovaries were compared. Findings that were frequent in the latter and strongly favored a metastasis were: 1) bilaterality, 2) microscopic surface involvement by epithelial cells (surface implants), and 3) an infiltrative pattern of stromal invasion. Findings that were less frequent but present exclusively or almost exclusively in metastatic carcinomas were: 1) a nodular invasive pattern, 2) ovarian hilar involvement, 3) single cell invasion, 4) signet-ring cells, 5) vascular invasion, and 6) microscopic surface mucin. Findings that were frequent in, and strongly favored, primary ovarian carcinoma were: 1) an "expansile" pattern of invasion and 2) a complex papillary pattern. Findings that were less frequent but also favored a primary tumor were: 1) size >10 cm, 2) a smooth external surface, 3) benign-appearing and borderline-appearing areas, 4) microscopic cystic glands, and 5) necrotic luminal debris. Findings that did not distinguish the tumors were: 1) a cystic gross appearance, 2) gross solid, papillary, necrotic, or hemorrhagic areas, 3) nature of cyst contents (mucinous vs nonmucinous), 4) stromal mucin (pseudomyxoma ovarii), 5) cribriform, villous, or solid growth patterns, 6) focal area resembling typical colonic carcinoma, 7) goblet cells, or 8) tumor grade. Primary and metastatic mucinous ovarian carcinomas can be distinguished from each other in the great majority of cases based solely on their conventional histopathologic findings. Careful gross evaluation is also important with special attention paid to the external surface of the ovarian tumor(s) to detect abnormalities that have the features of surface implants on microscopic evaluation.