KIT mutation has been implicated in sporadic mastocytosis, yet clusters in only a few sites in the molecule. For those malignancies associated with KIT mutation or over-expression, imatinib offers a specific therapeutic option, yet it has no effect on D816V mutation commonly seen in sporadic mastocytosis. The majority of cases of familial mastocytosis seem to lack KIT mutation. We report a kindred with mastocytosis in whom in vitro and in vivo sensitivity to imatinib was demonstrated. Mutation analysis of the KIT coding region in this family identified a novel A>T mutation at nucleotide 1547 [K509I] in exon 9 in both of the affected patients.