Long-term studies of morbidity and mortality in free-ranging primates are scarce, but may have important implications for the conservation of extant populations. Infants form a particularly important age group, because infant mortality can have significant effects on the demographics of slow-growing populations. Necropsies of mountain gorillas (Gorilla beringei beringei) have been conducted since 1968, and a comprehensive health-monitoring programme has been in place since 1986. For this study, gross post-mortem [n=70] and histopathology [n=53] reports for mountain gorillas under the age of 3.5 years (infants) in the Virunga massif were collated and reviewed, with the aim of providing the first comprehensive analysis of infant mortality and the histological evidence underlying morbidity in this species. Causes of morbidity and mortality were described, and compared by age, sex and over time. Trauma was the most common cause of death in infants (46%), followed by respiratory infections and aspiration (11%). Gastrointestinal parasitism (33%), altered lymphoid tissues (suggestive of viral infection on histology) (31%) and hepatic capillariasis (25%) were the most significant causes of morbidity. We also identified histological evidence for several currently un-described causes of morbidity in infant apes, including left atrial appendage inversion, fungal pneumoniaresembling adiosporomycosis,and renal tubular dysplasia. This study identified trauma, both natural (e.g. infanticide) and anthropogenic (e.g. snare entrapment), to be the most common cause of death in infant mountain gorillas. Anthropogenic factors are also likely important in deaths due to respiratory disease. Identifying the causes of mortality and morbidity in infants of this critically endangered species will help to inform policy aimed at their protection and guide ante- and post-mortem health monitoring in the future.