Several species of imported and captive-bred southeast Asian cockatoos with feather and beak disease (FBD) were evaluated. In recently emerging stained feathers from affected birds, intracytoplasmic magenta to basophilic inclusions of various sizes were found in macrophages and basophil-like cells within the pulp and feather epidermis. Occasionally, amphophilic intranuclear inclusions were seen within degenerated feather epidermal cells. On the basis of electron microscopic findings, intracytoplasmic inclusions were not membrane bound and consisted of crystalline arrays of viral particles (17 to 22 nm in diameter). On the basis of size and conformation, viral particles most closely resembled those of parvovirus or picornavirus. Consistent hematologic or serum enzyme differences were not found among affected or healthy cockatoos. Compared with findings in healthy cockatoos, cockatoos with FBD had significantly lower serum protein concentrations, and results of serum protein electrophoresis indicated that birds with FBD had significantly lower concentrations of prealbumin and gamma-globulin fractions. Mean pre- and post-ACTH plasma corticosterone concentrations of cockatoos with FBD were not significantly different from those of healthy cockatoos. In 8 of 9 affected cockatoos evaluated, serum T4 concentrations before and after thyrotropin stimulation were considered normal.