Spain holds > 95% of the European breeding population of the Eurasian griffon vulture. Vultures provide important ecosystem services in carcass removal and influence emissions of greenhouse gases. Despite the known toxicity of the non-steroidal anti-inflammatory drug diclofenac to this species and other vultures, in March 2013 the Agencia Española de Medicamentos y Productos Sanitarios (AEMPS) approved the use of two medicines containing diclofenac for veterinary use in horses, pigs and cattle in Spain. To assess the potential impact of medicated ungulate carcasses on Eurasian griffon vulture populations in Spain, information was gathered on the metabolism and elimination of diclofenac from medicated cattle and pigs. Secondly, probabilities of the death of a vulture per meal were calculated based upon experimental studies of diclofenac toxicity. Finally, annual numbers of vulture deaths expected to be caused by diclofenac were obtained by multiplying the death rates per meal by the estimated numbers of vulture meals taken from expected numbers of medicated carcasses suggested by AEMPS. Assuming that vultures feed on carcasses that were treated with diclofenac 8 h before the animal's death, the annual number of vulture deaths caused by diclofenac was estimated at 715–6389. Using a density-independent simulation model of a vulture population, the expected rate of decline of the Spanish population of Eurasian griffon vultures caused by these deaths is 0·9–7·7% per year. Due to the possibility of causing an important impact on vulture populations, findings justify a precautionary ban on the veterinary use of diclofenac in Spain and encourage the use of meloxicam, a vulture-safe alternative drug. A programme of monitoring of non-steroidal anti-inflammatory drug contamination of ungulate carcasses available to vultures and of moribund and dead obligate and facultative avian scavengers would be needed to be confident that a damaging level of contamination is not present. informacion[at]ebd.csic.es: Green et al (2016) Potential threat to Eurasian griffon vultures in Spain from veterinary use of the drug diclofenac. J Appl Ecol  53: 993–1003. doi:10.1111/1365-2664.12663