During the 1990s, populations of two species of griffon vulture, the Indian white-backed Gyps bengalensis and the long-billed Gyps indicus, declined by more than 90% throughout India. These declines are continuing and are due to abnormally high rates of both nesting failure and adult, juvenile and nestling mortality. Affected birds exhibit signs of illness (neck drooping syndrome) for approximately 30 days prior to death. Epidemiological observations are most consistent with an infectious cause of this morbidity and mortality. To investigate the cause of these declines, 28 vulture carcases, including adults and juveniles of both species, were examined in detail. Significant post-mortem findings included visceral gout, enteritis, vasculitis and gliosis. Although we have not yet been able to identify the causative agent of the declines, the results of our pathological studies are most consistent with those for an infectious, probably viral, aetiology. We examine hypotheses for the cause of the declines and, based on our epidemiological and pathological findings, we show infectious disease to be the most tenable of these.