The goal of this study was to assess various hematological variables in 10 cows after experimental infection with Ehrlichia phagocytophila. Blood samples were collected at regular intervals for examination of leukocytes for Ehrlichia organisms and for determination of hematological and biochemical variables. In addition, PCR amplification was performed throughout the disease period on blood and milk samples for the detection of E. phagocytophila organisms. The time of seroconversion and the duration of serum titers indicating positivity were determined by indirect immunofluorescence. For all cows, E. phagocytophila organisms were first detected microscopically in leukocytes 5 to 8 days postinfection and could be demonstrated for a period of 6 to 14 days. For all cows, the appearance of E. phagocytophila organisms in leukocytes coincided with transient erythropenia, leukopenia, and thrombocytopenia and a decrease in hematocrit and hemoglobin concentration. For five lactating cows, E. phagocytophila organisms were identified in leukocytes of milk samples during the acute phase of the disease, which, we believe, has not previously been reported. E. phagocytophila DNA was detected in blood samples by nested PCR from 1 to 2 days before to 2 to 12 days after the organisms were identified microscopically. In milk samples, E. phagocytophila DNA was detected for an average of 11 days.