Although not specific to KTS, a somatic mosaic activating mutation in the gene PIK3CA is suspected.4) The estimated incidence of KTS is 1–5/100,000.5–7) KTS with arterial aneurysms is much rarer, and only 17 cases have been previously reported (Table 3).8–24) In 6 of 18 cases, including ours, arterial aneurysms developed in the affected lower limbs.14–17,23) This paper aims to discuss nosological problems of arterial aneurysms associated with KTS. This evidence concerns the gene PIK3CA and angioosteohypertrophic syndrome.