Clinical, hematological, histopathological and parasitological studies on "Kasen" of horses in Hokkaido were made by the authors. The results thus obtained may be summarized as follows : 1. The disease made its appearance from early May to about the middle of August ; symptoms became worse in summer, but they disappeared gradually with cool season and naturally disappeared in winter. 2. The first attacks of this disease were usually upon 2 to 3 years old animals, while the range was as wide as 1〜15 years. Attacks were repeated every year and some of the animals showed 12 relapses. 3. This disease had nothing to do with species, sex, nutrition, breed or managment nor was stable infection confirmed. 4. The patients were all pastured, and the tendency of increase in occurrences was observed in areas near rivers and swamps, but relationship with blood-sucking insects has not yet been clarified. 5. Attacked regions were found mostly in the long hair parts. Characteristic symptoms were severe itchiness and secondary eczema or dermatitis such as depilation, dandruff, erosion, ulcer, hyperkeratosis and crust formation as a results of the animals rubbing themselves. 6. In the bacteriological examinations in affected regions, Staphylococcus and Streptococcus were found in almost all cases, but no other special pathogenic microorganisms were detected. 7. Hematological examination showed that erythrocyte and leucocyte numbers were all within normal range, but eosinophiles were increased in most cases. 8. Living microfilariae (0.22〜0.24×0.0039〜0.0045 mm) were detected from pieces of cutaneous tissues in 4 patients (11%). These microfilariae are not yet identified, because they were too few to examine their body structure in detail. 9. Histopathologically, 17 cases (53.1%) showed eczematous lesions and infiltrations of eosinophiles, however, microfilariae were not found in all cases. 10. Treatments with intravenous injections of antimonyl derivates (Stinal and Neostinal) did not obtain beneficial results
