The outbreak of the disease occurred in a large multiple-age farm with about 50,000 meat turkeys, where groups of 6-8000 one-day-old birds were stalled up every 14 days. All the turkey poults housed were affected mostly in the 1.-3. week of the life. The respiratory disease spread rapidly within the flocks and were characterised clinically by inclination of huddle, ruffled feathers, anorexia, stunted growth, swelling of the infraorbital sinus and nasal discharge. The clinical apparent disease lasted 3 to 4 weeks on the average in the affected flocks and were associated with a mortality from 7-20 percent. The main pathoanatomical lesions were catarrhal-fibrinopurulent rhinitis, sinusitis, tracheitis, bronchopneumonia and air sacculitis as well as atrophy of the thymus. Fibrinous adhesive peri- and epicarditis, perihepatitis, miliary necrotic foci in the liver and diarrhea have been found less frequently. The results of cultural and serological examinations of moribund and dead turkey poults of 6 different flocks indicate that Bordetella avium and Chlamydia psittaci are the primary inciting agents of the respiratory disease. However, the following severe course of the disease were mainly caused by concurrent infections with Klebsiella pneumoniae subsp. pneumoniae, Escherichia coli and Pseudomonas fluorescens. In some cases coccidiosis with lesions in ceca were additionally diagnosed. Campylobacter jejuni could be always isolated culturally from the liquid cecal content of diseased birds.