Recently, increased incidence of outbreaks of infectious laryngotracheitis (ILT) was observedvamong chicken flocks in Egypt. These werevassociated with variable high mortalities.vInvestigation of 8 such outbreaks revealed that three nonvaccinated replacement layer pullets of 6-8 weeks of age were severely affected with mortality rates of 11-12% up to 40%, while in a 4^th pullet flock of 24 weeks of age, which was comparatively milder and resulted in only 2 % mortality. On the other hand, in four commercial broiler flocks of 5-7 weeks of age with no history of previous vaccination, the disease varied in severity and mortalities (7.6 - 18.0%). Eight isolates were recovered from these outbreaks and were identified as those of ILTV. Pathogenicity tests for two representative isolates from pullets and broilers were carried out by inoculation of each intratracheally into  susceptible chickens of the respective types (egg and meat-type), morbidity and mortality rates were used to calculate an intratracheal pathogenicity index used the same as that described for Newcastle disease virus, in addition to microscopic tracheal lesion scoring as criteria for judging their pathogenicity . Results indicated that both isolates were pathogenic like wild ILT field viruses. Reversed virulence of modified live vaccine viruses was speculated under prevailing conditions of suboptimal management practices, hygiene and biosecurity measures which help spread of infection between flocks, beside the role of latently infected carrier birds and other factors in the epizootiology of the disease, especially in nonvaccinated flocks, were discussed.