In the present work, chickens and turkeys were infected with virulent or attenuated Histomonas meleagridis to investigate and compare the effect of both isolates on birds. Thereby, histomonads of a clonal culture were propagated in vitro either for a short period of time (21 passages) to preserve virulence or for 295 passages to achieve attenuation. On the first day of life birds of each species were infected with either virulent or attenuated parasites. Throughout the experiment, all birds were examined daily for clinical signs attributable to the infection. Furthermore, the excretion of viable parasites was determined after in vitro reisolation from cloacal swabs. For the investigation of pathological changes of organs a defined number of infected birds were killed on d 4, 7, 10, 14, and 21 postinfection (PI) and necropsy was performed. By this routine, changes in livers and ceca were classified by a scoring system to evaluate the severity of lesions. Samples of cecum, liver, and lung were generated and screened for the presence of parasites by PCR and immunohistochemistry. Turkeys infected with virulent histomonads showed first clinical manifestation of histomonosis on d 10 PI, whereas the remaining birds did not express clinical signs. Positive reisolations of virulent and attenuated histomonads were obtained intermittently from individual chickens and turkeys from d 2 PI until the end of the experiment. Both species of birds displayed lesions in the ceca and the liver following infection with virulent parasites, whereas no changes occurred in birds inoculated with attenuated histomonads. The PCR revealed the dissemination of virulent histomonads in ceca, livers, and lungs of some chickens and turkeys in contrast to attenuated parasites, which were exclusively found in cecal samples. The attenuated isolate of H. meleagridis did not induce clinical signs or pathological changes and offers high safety after infection of chickens and turkeys. Therefore, the in vitro attenuation and the use of avirulent histomonads represent a viable tool for vaccination against histomonosis.