The incidence and histopathology of the spontaneous tumors in over 2-year-old ddY strain mice, an outbred widely used in Japan, are described. The incidences of the total spontaneous tumors were 47.5% in males and 66.7% in females in actual numbers. These, however, could be corrected to 64.4% and 80.0%, respectively, when the accidental death are omitted from the results. The incidences of the pulmonary tumors were very high; 39.2% in males and 26.7% in females of the total spontaneous tumors, respectively. Other tumors which showed high incidences were, lymphoid tumors; 17.5% in males and 19.2% in females and mammary tumors; 26.7% exclusively in females. However, no strain-specific tumors were observed which are common in inbred strains such as leukemia in AKR or mammary tumors in C3H strains, partly suggesting broad genetic variation of the strains. Histopathological examination revealed prevailing types of the tumors to be papillary adenoma of pulmonary tumors, simple adenoma of mammary tumors and poorly differentiated lymphosarcoma of lymphatic tissues.