Two forms of neuronal ceroid lipofuscinosis (CLN) are enriched in the Finnish population: the infantile form (CLN1), which is the most common progressive encephalopathy of small children, and the variant late infantile form (variant CLN2), which is a rare, atypical form of neuronal ceroid lipofuscinosis. We recently established the linkage of the infantile form (CLN1) to the short arm of chromosome 1 close to the anchor marker D1S7. Here we demonstrate a linkage disequilibrium of CLN1 chromosomes using the two closest markers, DIS62 and L-MYC at the short arm of chromosome 1 (P less than 0.0025). The results of linkage analyses in Finnish variant CLN2 families using the markers linked to CLN1 revealed an exclusion; i.e., this form of CLN is caused by a locus different from that of CLN1. This finding was confirmed with the result of the M-test for heterogeneity. The genealogical data collected further support the molecular genetic findings and provide evidence that the mutation causing CLN1 in Finland is very old, whereas the mutation causing the variant CLN2 could be a result of a younger, i.e., more recent founder effect.