A large epidemiological survey of inhospital chronic hemodialysis patients was conducted in 27 (93%) of the 29 dialysis centers in Michigan. Serum was collected from 699 patients on chronic maintenance hemodialysis for periods from one month to eight years. Hepatitis B surface antigen (HBsAg) was determined in all patients by radioimmunoassay and positive samples were confirmed by specific neutralization. Antibody against HBsAg (anti-HBs) was determined by radioimmunoassay in 110 HBsAg negative patients from six dialysis units with a high prevalence of hepatitis B. HBsAg was detected in 80 (11.4%) patients distributed among 21 (78%) of 27 dialysis units and anti-HBs in 34 (31%) patients from the selected dialysis units. The prevalence of HBsAg was related to duration of dialysis, number of blood transfusions, and to a history of bilateral nephrectomy, but not to age, sex, race, nor the underlying renal disease. Twenty-one (26%) of the 80 HBsAg positive patients had not been previously identified by the clinical laboratories of their institutions. Since preventive measures were not taken in the care of these inapparent carriers of HBsAg, they represent an unrecognized risk.