The goal of this study was to determine the clinical signs and course of disease in five lactating cows and in five dry cows after experimental infection with Ehrlichia phagocytophila. Ten clinically healthy Swiss Braunvieh cows, seronegative for E. phagocytophila, were injected with 50 ml of whole blood containing E. phagocytophila. The cows were examined daily for 21 days, and blood samples were collected for microscopic examination of leukocytes for the infective agent. All cows became ill with symptoms of tick-borne fever after an incubation period of 5 to 9 days. The most important clinical signs were pyrexia (40.2-41.7 degrees C), decreased milk production and mildly to moderately disturbed general condition. In addition, there were respiratory symptoms such as polypnea, nasal discharge, cough and abnormal lung sounds. Clinical signs returned to normal in all cows without treatment after an average of 8 days. E. phagocytophila bodies were seen in leukocytes 5-8 days after infection and were present for 6-14 days. The course of disease was more severe in dry cows than in lactating cows. It can be concluded that experimental infection of cows with E. phagocytophila generally has a mild course. However, the associated decrease in milk production may be of economic importance.