Total serum creatine kinase (CK) and its isozyme activities were determined in dogs with dirofilariasis. Before heartworm removal, total CK and isozyme activities in dogs of the mild group were not different from those in dogs of the heartworm-free group. BB activity was higher in dogs of the hemoptysis group. Dogs of the ascites group displayed a mild increase in MM activity. In dogs of the caval syndrome (CS) group, total CK and MM activities were highest among the heartworm-free and heartworm-infected dogs, and MM isozyme accounted for most (75%) of total CK activity. MB and BB activities were also higher. However, there were no significant differences in CK activities between the surviving and non-surviving cases. In dogs with pulmonary heartworm disease (mild and ascites groups), MM activity correlated significantly with the number of heartworms (r=0.45), hematocrit value (Ht, r=-0.40), serum alanine aminotransferase (ALT, r=0.42) and lactate dehydrogenase (LDH, r=0.46) activities, mean pulmonary arterial pressure (r=0.64) and total pulmonary resistance (r=0.50). In dogs with CS, MM activity did not correlate with any parameter, but BB activity correlated with the number of heartworms at the right atrium (r=0.61), Ht (r=-0.53), ALT (r=0.80), LDH (r=0.73) and serum urea nitrogen (r=0.47). At 1 week after heartworm removal, BB and MM activity decreased in dogs of the hemoptysis and ascites groups, respectively. In dogs of the CS group, total CK and MM isozyme activities decreased markedly (P<0.01) regardless of their prognosis.