20α-hydroxysteroid dehydrogenase (20α-HSD) activities in 57 neoplastic tissues surgically removed from dogs were measured. Forty-eight of 57 tumor samples were shown to possess 20α-HSD activity. These tissues were histopathologically classified into 22 benign and 26 malignant tumors. Among these tumors, mixed tumor types demonstrated the higher 20α-HSD activity than epithelial and non-epithelial types, and malignant tumors of each tissue type showed slightly but not significantly higher 20α-HSD activities comparing with the corresponding benign ones. Comparing with the activity of normal tissues examined, the corresponding tumor tissues showed significantly higher 20α-HSD activities. Thus, 20α-HSD activity was found in neoplastic tissues at a considerable high rate and the activity seemed to be higher in pathologically malignant tumors.