In the winter of 1993–94, house finches (Carpodacus mexicanus) with severe conjunctivitis (later shown to be caused by Mycoplasma gallisepticum) were first observed in suburban Washington D.C. (USA) and adjacent states. Using a large network of volunteer observers in eastern North America, we were able to track the monthly prevalence of the disease between November 1994 and March 1997. Using the information on 24,864 monthly data forms, we describe the very rapid spread of the conjunctivitis epidemic through the eastern house finch population. The epidemic first expanded mainly north, probably carried along by house finches on their return migration, then mainly toward the southeast, and later west. By March 1997, conjunctivitis had been reported from most of the eastern range of the house finch. The prevalence of the disease seemed to fluctuate seasonally with increases in the fall, probably as a result of dispersing juveniles. House finch numbers decreased throughout winter in areas with cold winters and high conjunctivitis prevalence, suggesting significant mortality associated with the disease.