We determined the underlying aetiology of blindness for the registered blind population of the Province of Newfoundland and Labrador. In both 1981 and 1984 single-gene disorders accounted for 30% of total blindness and congenital defects for another 10-11%. Genetically determined conditions, diabetes, and senile macular degeneration (SMD) were the three leading causes of registration in each year, 1980-4. We calculated mean ages of registration and mean ages of death over the last four years for five major aetiological groups. Patients with genetic conditions were registered at a much younger age and had a correspondingly longer duration of blindness (21 years as compared with 5 years for either diabetes or SMD). Total 'person-years of blindness' was then calculated from the product of this duration of blindness and the total numbers registered in each group. This index shows that the overall individual and population impact of monogenic blindness is overwhelmingly greater than that of other causes (6849 person-years compared with 270 for diabetes and 430 for SMD). In view of this frequency and duration of monogenic blindness, and also of the substantial hereditary liability to relatively common causes of blindness such as glaucoma, diabetic retinopathy, and high myopia, we suggest that more attention needs to be paid to elucidating the genetic contribution to blindness.