Isolation rates of Campylobacter spp. were analyzed for an 8-year period, 1978 through 1985. Three laboratories of the Ontario Ministry of Health examined 146,842 human feces samples for bacterial pathogens, including Campylobacter jejuni and Campylobacter coli. Campylobacter spp. were isolated from 5,580 specimens (3.8%), with monthly isolation rates ranging from 1.1 to 7.4%. The data showed a seasonal distribution of isolations, with peaks during the summer months (June to September). Most infections were in children, adolescents, and young adults. More males were infected than females; this finding was most pronounced in the age groups under 25 years. In Northern and Central Ontario, a strikingly higher incidence was observed among farm residents than among rural nonfarm or urban residents. Of 89 farm residents, 63 had consumed raw milk (61 bovine, 2 goat) within 72 h before becoming ill.