In 1972 there were outbreaks of eastern equine encephalitis in the Eastern Townships, Quebec, Canada and in Connecticut, USA. Climatic data including Northern Hemisphere synoptic charts were examined. The findings indicate that the virus could have been brought to Lac Brome by infected mosquitoes carried on surface winds from Meriden, Connecticut, on the night of August 22-23, 1972. The distance of 400 km would have been covered in 14-16 h at a speed of 25-30 km h-1 and at a temperature of 15 degrees C and higher. The first case was recorded 13 days later on September 5, 1972. The outbreak at Meriden, Connecticut started on August 21, 1972. On August 7, 1972 southwesterly winds blew along the Atlantic coast at heights up to 1.5 km. Infected mosquitoes could have been carried on the wind from Cape May, New Jersey, Delaware-Maryland-Virginia peninsula, North Carolina or Georgia. Flights would have been at 17 degrees-20 degrees C and lasted 5-6, 9-10, 14-16 and 20-26 h depending on the origin. The arrival on August 8, 1972 coincided with a cold front moving from the northwest through Connecticut. Culiseta melanura is regarded as the mosquito species most likely to have been involved in the transmission of infection.