PURPOSE: To estimate the number of cancer cases during 2001, in Korea, through a nationwide hospital based cancer registration by the Korea Central Cancer Registry (KCCR). MATERIALS AND METHODS: One hundred and thirty four hospitals participated in the KCCR program in 2001. Cancer cases were coded and classified according to the International Classification of Diseases for Oncology 2nd edition (ICD-O-2). The software program "IARC Check" was used to evaluate the quality of the registered cancer cases. Of the 111, 816 malignancies registered, 10, 106 (9.0%) duplicated malignancies were excluded. Among the remaining 95, 542 malignancies, 3, 598 (3.8%) cases with carcinoma in situ (Morphology code/2) were separated. Finally, 91, 944 malignancies were analyzed. RESULTS: Of the total 91, 944 malignancies, 51, 753 (56.3%) cases were males and 40, 191 (43.7%) were females. More than one third of cases were from the elderly (65 years old and more). The six leading primary cancer sites, in the order of their relative frequency, were stomach (24.1%), followed by the lung (16.0%), the liver (16.0%), the colorectum (10.5%), the bladder (3.4%), and the prostate (2.8%) among males. In females, the breast (16.1%) was the common cancer site, followed by the stomach (15.3%), the colorectum (10.5%), the cervix uteri (10.1%), the thyroid gland (8.3%) and the lung (6.6%). CONCLUSION: With the continued increase in cancer cases, the total number of registered cancer cases in Korea continues to rapidly increase.