To determine the prevalence of refractive errors in the young and middle-aged adult population in Norway. Refractive errors were measured in a population-based sample of young (20-25 years) and middle-aged (40-45 years) adults participating in the Helseundersøkelse i Nord Trøndelag (HUNT) Health Study, conducted in the County of Nord-Trøndelag in Norway. A total of 3137 persons (1248 young and 1889 middle-aged adults) with corrected visual acuity > or = 0.5 (in either eye) were included in the study. The prevalence of myopia was 35.0% in the young adult group and 30.3% in the middle-aged group. Myopia was significantly higher in women aged 20-25 years (36.4%) than in men aged 40-45 years (28.1%). Prevalence of hyperopia increased with age from 13.2% (20-25 years) to 17.4% (40-45 years). The highest rate of hyperopia (20.1%) was encountered in middle-aged women. The results show a slightly higher prevalence of myopia in the general population of Norway than previously estimated.