The onset and course of disease in a captive herd of Rocky Mountain bighorn sheep (Ovis canadensis) were studied. Major clinical signs were diarrhea, persistent coughs, mucopurulent nasal discharges, loss of weight, poor pelage appearance, and delayed pelage shedding. Primary pathological findings in 16 of 17 deaths were related to pneumonia. Mycoplasma arginini, Pasteurella sp., and Streptococcus sp. were isolated and considered probable etiologic agents. Parasites were not considered to be a primary cause of the clinical signs or the lung disease observed. Hematological changes indicated the chronicity of the disease and had prognostic and limited diagnostic value. Amyloidosis was observed in seven animals, suggesting a high susceptibility of the bighorn to secondary amyloidosis.