Five of the six families with the Berardinelli-Seip syndrome in Norway cluster in six adjacent rural municipalities of south-western Norway. The six patients from this area were born between 1951 and 1973, none between 1974 and 1995. The absence of new cases may be explained by a decrease in the intraregion marriage rate and inbreeding. Genealogical investigations show that the mutation must have occurred at least 400 years ago. The sixth family was clinically different and geographically sporadic from a Finnish-descent rural East Norwegian population. A genetic linkage study of all six families revealed fresh crossovers versus the disease allele in nine DNA marker systems and the absence of recombination in three (maximum lod score + 1.3). None of the last showed allelic association. These families are included in an international effort to map the CLBS locus. The patients have been included in the homozygosity testing of totally 28 patients in an international collaborative study. The three patients, assumed identical in descent from both parents, were jointly homozygous in none of the 250 dinucleotide markers tested. A heterochromatic 9qh + segregated from one parent in two families.