Gaucher disease type III (GD) is found at a high frequency in northern Sweden. The contemporary Swedish index families are found in two geographically distinct clusters with the highest world-wide frequency of type III GD. A single T-to-C transition in exon 10 has previously been identified in patients from one of the two isolates and we report there the same mutation in the second isolate. Mutational analysis was combined with a genealogical reconstruction of 19 contemporary index families. Both clusters were traced back to two corresponding pairs of ancestors over a 9-13 generation span. Molecular studies show that the two clusters are compatible with a single founder who arrived in northern Sweden in or before the 16th century.