Persistent hepatitis B infections among patients and frequent new hepatitis B infections among both patients and staff were a major problem in our dialysis unit during its first two and one-half years of operation. During this time the mean quarterly rate of conversion to HBsAg positivity among patients ranged from 0-60% (mean 12%); in staff it ranged from 0-13%. Control efforts, including strict temporal isolation, improved sanitary measures, and use of parallel plate dialyzers without geographic or machine isolation, were begun late in 1976. After a four-month lag, new HBsAg conversions ceased among the 30 patients and staff at risk, despite continued dialysis of eight HBsAg-positive patients (at least four of whom were HBeAg-positive). Over the succeeding three years the conversion rate was zero in both patients and staff. This experience suggests that conservative control measures without geographic separation of patients may be sufficient to control an established outbreak of hemodi-alysis-related hepatitis B. Controlled prospective trials of this hypothesis are warranted.