Infection with the intracellular microsporidium Encephalitozoon cuniculi can cause a serious disease-encephalitozoonosis in various animals and people. Several species of mammals, including the horse, were seem to be potential sources of encephalitozoonosis for animal as well as humanhosts. The disease diagnosis is based on clinical signs, pathological findings, and the detection of E. cuniculi or circulating antibodies directed against the parasite. This study investigates the seroconversionto E. cuniculi in horses admitted to the Veterinary Teaching Hospital of the Hebrew University of Jerusalem and 3 different private horse-riding farms across Israel. Antibodies to E. cuniculi were determined usingthe IFA test in the sera from 102 horses. Of 72 asymptomatic horses, 60% were seropositive and 19% ofthe positive samples showed a titter of 1:512. Of 30 horses with various clinical signs, 80% were seropositive and 68% of the positive samples showed a titer of 1:512. High titers were associated with colic and neurologicalsigns. This could prove to be interesting if the high percentages of prevalence of antibodies level inhorses are an indication of health risk in humans.