The point mutation of c-K-ras oncogene at codon 12 was investigated in 15 cases of mucin-producing cystic tumor of the pancreas by a modified polymerase chain reaction (PCR) method. Three percent formalin-fixed and paraffin embedded materials were employed for the present investigation. These fifteen cases were histopathologically classified into five categories (from group I to V) according to Kozuka's classification which was based on cell atypia of the tumor cells; 5 cases were judged as group V, 4 cases as group IV, and 6 cases as group III. The frequency of point mutation was 5/5 in group V, 3/4 in group IV, and 2/6 in group III. The frequency of point mutation was correlated with morphologically determined cell atypia. Among group III, the point mutation was noticed in the cases with larger tumor size. From genetical point of view, the results indicated more malignant biological feature of mucin-producing cystic tumor of the pancreas than has generally been accepted.