A disease outbreak investigation was conducted in western Queensland to investigate arare suspected outbreak of pyrrolizidine alkaloid (PA) toxicosis in horses. Thirty five of132 horses depastured on five properties on the Mitchell grass plains of western Queens-land died in the first six months of 2010. Clinical–pathological findings were consistentwith PA toxicosis. A local variety of Crotalaria medicaginea was the only hepatotoxic plantfound growing on affected properties. Pathology reports and departure and arrival dates oftwo brood mares provided evidence of a pre wet season exposure period. All five affectedproperties experienced a very dry spring and early summer preceded by a large summerwet season. The outbreak was characterised as a point epidemic with a sudden peak ofdeaths in March followed by mortalities steadily declining until the end of June. The esti-mated morbidity (serum IGG > 50 IU/L) rate was 76%. Average crude mortality was 27% buthigher in young horses (67%) and brood mares (44%). Logistic regression analysis showedthat young horses and brood mares and those grazing denuded pastures in December weremost strongly associated with dying whereas those fed hay and/or grain based supplementswere less likely to die. This is the first detailed study of an outbreak of PA toxicosis in cen-tral western Queensland and the first to provide evidence that environmental determinantswere associated with mortality, that the critical exposure period was towards the end ofthe dry season, that supplementary feeding is protective and that denuded pastures andthe horses physiological protein requirement are risk factors.