A fatal adenoviral infection was recently recognized in suckling Arabian foals from Colorado, Kansas, Nebraska, and Wyoming. Affected foals had clinical signs of pneumonia accompanied by fever, absolute lymphopenia, and neutropenia. Characteristic lesions with typical adenoviral inclusions occurred in respiratory epithelium of all cases. Other tissues with significant changes and inclusion bodies were conjuctiva, urinary mucosa, pancreas, and salivary glands. Lymphoid follicular atrophy and hepatic degeneration were frequent but were not accompanied by inclusions.