Nearly one-third of adults aged ≥25 years in Australia have vitamin D deficiency, according to an evaluation of 11,247 adults who were enrolled in the AusDiab study. The prevalence of vitamin D deficiency (defined as a serum concentration of 25-hydroxyvitamin D <50 nmol/l) was 31% in this population. Vitamin D deficiency was more prevalent in women than in men (39% versus 22%) and was increased in people of non-European ancestry, in those with obesity or low physical activity, in elderly individuals and in those with a high level of education. Prevalence also increased in winter–spring compared with summer–autumn.