Three reports previously overlooked describe an encephalitis of horses that occurred in three coastal counties of Massachusetts in 1831. The disease appeared in midsummer, reached its peak in August and disappeared by the end of September. Mortality was high; of 100 horses known to be affected near Middleboro in Plymouth County, 75 died. Death usually occurred within one to four days after signs appeared. The affected horse first appeared dull and tended to move in a circle. Incoordination became marked as the disease progressed. Spastic paralysis, prostration and coma usually preceded death. Some animals that survived regained their health but others were killed because restoration to usefulness seemed unlikely. The only lesion seen on necropsy was a slight inflammation of meninges. The disease attacked animals of all ages. It was common among horses on pasture and rare in those kept stabled. The disease described in 1831 fits modern descriptions of Eastern equine encephalomyelitis.