The infantile form of neuronal ceroid lipofuscinosis (INCL) is a progressive encephalopathy in children < 2 years old. The disease is one of the Finnish diseases, enriched in this genetically isolated population. The gene responsible for INCL has been recently assigned to the short arm of human chromosome 1. Here we describe DNA-based prenatal and carrier diagnostics using a highly polymorphic marker (HY-TM1) which demonstrates a strong allelic association to the disease locus. 88% of Finnish INCL patients were observed to have the same affected genotype, suggesting that one major CLN1 mutation is enriched in this population. In contrast, all the non-Finnish INCL patients had different allele combinations.