The epidemiology of neuronal ceroid lipofuscinoses (NCL) in Scandinavia was studied. For juvenile NCL 40 Swedish living patients were identified. The corresponding number for Finland was 61, for Norway 28, for Denmark 16 and for Iceland three. The prevalence of juvenile NCL was thus 4.6, 12.2, 6.5, 3.1 and 11 per million inhabitants in Sweden, Finland, Norway, Denmark, and Iceland, respectively. For calculating incidence the years 1976-85 were used. The incidence was 2.2 per 100,000 live births in Sweden, 4.8 in Finland, 3.7 in Norway, 2.0 in Denmark, and 7.0 in Iceland. Late infantile NCL was found in five Swedish children, including one variant form, CLN5. This gives a prevalence of about 0.6 per million. In Finland 13 cases gave a prevalence of 2.6 per million with the variant form in 80% of cases. In Norway, three children corresponded to a prevalence of 0.7 per million and one case in Iceland to 3.8 per million. No Danish cases were reported. As for infantile NCL, sixteen Swedish cases have been diagnosed during the 27-year period 1968-95, six are presently alive. This gives an estimated prevalence of 0.7 per million and an incidence of 0.6 per 100,000. The prevalence and incidence of infantile NCL in Finland were 5.4 per million and 5 per 100,000, respectively. The prevalence in Norway was 0.2 per million inhabitants. The variability of onset, clinical course and symptoms in the Swedish cases of juvenile and infantile NCL were analysed.