To evaluate the clinical usefulness of tissue and serum gelatinase activity as a prognostic marker of canine tumors, tissue samples from 60 tumors and corresponding serum samples from the same animals were collected at the time of biopsy and surgery. On the basis of histopathology and clinical aggressiveness of metastasis and recurrence (MR), the cases were divided into 6 categories: non-inflammatory (Inf(-)) and inflammatory (Inf(+)) benign, and the Inf(-) MR(-), Inf(-) MR(+), Inf(+) MR(-), and Inf(+) MR(+) malignant. Gelatinase activity was determined semi-quantitatively using gelatin zymogram with a gelatinase standard from cultured canine peripheral blood mononuclear cells stimulated with lipopolysaccharide. No significant difference in gelatinase activities in tissue extracts was evident between the benign and malignant tumors. Inf(+) benign tumors, as well as Inf(-) MR(+), Inf(+) MR(-) and Inf(+) MR(+) malignant tumors, showed significantly higher tissue gelatinase activity than Inf(-) benign. The tissue activity in Inf(-) MR(-) malignant was significantly lower than in Inf(+) MR(-) and Inf(+) MR(+) malignant. The serum activity was significantly higher in the malignant cases than in the control and the benign. Inf(-) MR(+), Inf(+) MR(-) and Inf(+) MR(+) malignant tumors induced significantly higher gelatinase activity in serum than Inf(-) benign tumors. Gelatinase activity in serum was positively correlated with that in tumor extracts. Increased gelatinase in tumor tissue and serum may be correlated with inflammation as well as tumor aggressiveness, and thus should be used in combination with histopathology for predicting tumor metastasis or recurrence.