We report the prevalence of diabetes in a population sample of 10,083 persons who were aged 25 to 64 years in eight city centres. One hundred and sixty-nine persons were known to have diabetes and 48 persons were newly-discovered to have diabetes on the basis of fasting hyperglycaemia (plasma glucose level, equal to or greater than 7.8 mmol/L). Type-1 diabetes was identified by clinical criteria and accounted for 19% of cases of known diabetes, but this proportion ranged from 67% of persons with known diabetes in the age-group of 25-29 years, to between 9% and 15% in the age-groups of persons who were 50 years of age and over. For both known and newly-discovered cases, persons with type-2 diabetes showed a significant male preponderance, which suggests that an environmental factor is operating preferentially on male subjects to cause this form of diabetes. The results of the present study, when taken with those of other recent prevalence studies, enable an approximate estimate of numbers of persons with diabetes in Australia. Approximately 250,000 persons have diagnosed diabetes, of whom 40,000 persons have type-1 diabetes, including 7000 persons who are less than 25 years of age. The number of persons with undiagnosed diabetes who could be identified on the basis of fasting hyperglycaemia is estimated to be 75,000; an additional 150,000 persons would be diagnosed to have diabetes if they were to undergo glucose tolerance tests. This study gave prevalence rates for known diabetes in Australia in 1984 of 1.6% at all ages and 2.4% for adults who were 21 years of age and older; additionally, the estimated prevalence rates for undiagnosed diabetes were 1.4% for all ages and 2.2% for adults.