The cases of malignant ovarian tumors treated at Chiang Mai University hospital between 1992 and 2003 were histologically reviewed. The medical records, the radiologic findings, and the follow-up outcome in the cases suspicious or diagnostic of metastases were reviewed to confirm the diagnosis and to determine the primary sites. Metastatic tumors accounted for 30% of malignant ovarian tumors. A total of 170 cases of metastatic tumors included 117 cases with nongynecologic origin and 53 cases with gynecologic origin. Nongynecologic metastatic tumors were from large intestine (31%), stomach (14%), intrahepatic bile duct (10%), breast (9%), extrahepatic bile duct/gallbladder (7%), appendix (5%), hematologic tumors (3%), others (4%), and unknown primary site (16%). Metastatic gynecologic tumors were from cervix (53%), corpus (34%), fallopian tube (11%), and gestational trophoblastic disease (2%). The proportion of metastatic tumors to malignant ovarian tumors in northern Thailand was comparable to those of the Western or Japanese studies. However, the distribution of the primary sites was different and was correlated with the cancer incidence in Thai women. The majority of mucin-producing adenocarcinomas involving the ovaries were metastatic tumors.