During an outbreak of hemorrhagic fever with renal syndrome in the Anhui Province of China, a door-to-door survey was conducted of all of the 10,024 residents of eight village communities. The incidence rates were higher in males than in females and higher in adults than in children. Of all the residents surveyed, however, those who had slept on the ground or had been engaged in heavy farm work were at a significantly higher risk of illness than were those who slept on wooden beds or did light work. Among men who did heavy farm work and who gave a history of sleeping in straw huts on the drained swamp, the attack rate was 2.33 times higher than it was for those sleeping exclusively in their permanent homes. Among both sexes, threshers had a significantly higher attack rate than did nonthreshers. Suspicions that trombiculid mites or gamasoid mites act as vehicles of transmission cannot be accepted as an explanation for the association of the risk of illness with the type of work done and with sleeping arrangements. Circumstantial evidence supports our hypotheses of airborne transmission and transmission by inoculation of infectious materials into skin lesions.