Programmed cell death, or apoptosis, is involved in the normal physiology of many immunocompetent organs, including lymphocytes of the bursa of Fabricius in chickens. Involvement of apoptosis has also been described in some viral diseases such as AIDS. The purpose of this work was to study the potential role of apoptosis in the pathogenesis of Gumboro disease in the bursa of Fabricius. Our results show that 1-3 days after infection of young chickens with infectious bursal disease virus, the number of apoptotic cells increases and cellularity and proliferation decrease. Because of the dynamic nature of bursal lymphocyte populations and the involvement of apoptosis in lymphocyte cell physiology, the increased level of cells undergoing apoptosis may be due to an impairment in the withdrawal of apoptotic cells. A concomitant increase in macrophages in infected bursae and a dramatic decrease in cellularity suggest that an increase in apoptosis may be an important cause of cell depletion.