Purpose: To investigate patients under the age of 20 with aniridia in Sweden and Norway in order to estimate the prevalence of aniridia, to describe clinical signs and identify complications in the young, which will help improve diagnostic tools and treatment. Methods: A thorough search for patients with aniridia (of all ages) was performed. Sixty-two of the 181 patients were under the age of 20. Fifty-two of them were examined and they constituted the study population. Patient history was obtained and all participants underwent clinical ophthalmologic examination, including photography. Blood samples were taken for mutation analysis. Results: Epidemiological data are only based on the results in Sweden. The age-specific prevalence in Sweden was 1:47 000, male/female ratio was 0.57, mean age 12 years and median age 14 years. The proportion of sporadic cases including WAGR (Wilms tumour, Aniridia, Genitourinary abnormalities, Mental Retardation) and Gillespie syndrome (aniridia, cerebellar ataxia and mental retardation) was 48%. In the entire study population (Sweden and Norway), the mean visual acuity (VA) was 0.2 (range 0.04-0.9). We found VA < 0.3 in 80% and <0.1 in 18% of the patients. Twenty-two patients (42%) had one or more of the sight threatening complications such as cataract/lens luxation, corneal clouding or glaucoma. Conclusion: Descriptions of aniridia in the younger are rare. This study shows that aniridia seems to be more common than previously estimated and that some complications appear early in life. Watchfulness as regards these complications and regular examinations are essential even in the youngest.