Adenoviruses were isolated from broiler flocks with hydropericard syndrome in Ecuador and Pakistan. The isolates were identified using neutralizationtest, crossneutralizationtest and restriction enzyme analysis and proved to be fowl adenovirus serotype 4. The restriction enzyme analysis of the Desoxyribonucleic acid (DNA) showed no genetical differention among the six field isolates from Ecuador (K1013/1-6), however they varied from the isolate K31, isolated from flocks in Pakistan and from the reference strain of FAV4 (KR5). In experimental studies embryonated SPF-chicken-eggs were inoculated via the yolk sac with three different infectious doses of one field isolate from Ecuador and the field isolate from Pakistan. Both induced 100% embryo mortality. A reduced embryo mortality was obtained with the reference strain. Infection of one-day-old SPF-chicks with 105 TCID50 via oculo-oral routes with both field isolates produced characteristic clinical signs, gross and microscopic lesions of hydropericard syndrome accompained with 100% mortality. Oculo-oral infection of 3-weeks-old SPF-chicks, with both isolates failed to reproduce the disease. Also SPF-chicks of the same age showed no diseases after intramuscular inoculation with K31. Contrary, intramuscular infection with K31 and oculo-oral infection with both field isolates in the same age group SPF-chicks, that had previously been medically immuno-compromised, lead to 50% mortality accompanied with characteristic gross and microscopic lesions. Infection of 33-weeks-old SPF-parent flock with K31, via oral routes, resulted vertical transmission of the virus without any effect on hatchability. Neither clinical symptoms nor gross lesions could be observed in the progeny, even in those with no detectable maternal antibodies. The parent flocks showed in the second weeks post infection a high antibody titer. The virus excretion was periodical.