A 4-year-old boy had hemihypertrophy and hemihypertrichosis on his left leg since he was 10 months old. Any other family history or past medical history was not noted. The circumference of left calf was 2cm thicker than right calf, and magnetic resonance image also showed hypertrophic muscles. Folliscopic image revealed increased density and thickness of hairs on his left calf. No other systemic abnormalities were detected. The patient was diagnosed idiopathic hemihypertrophy with hemihypertrichosis. Hemihypertrophy is a rare congenital abnormality, showing a fast growth of one side of the body. It usually accompanies diseases such as Beckwith- Wiedemann syndrome, Silver-Russell syndrome, Proteus syndrome. Hypertrichosis shows excessive growth of body hair over the normal range. Three types of congenital hypertrichosis were reported, congenital hypertrichosis lanuginosa, congenital hypertrichosis terminalis and nevoid hypertrichosis. Nevoid hypertrichosis is similar to hypertrichosis in our patient. Co-presence of hemihypertrophy and hemihypertrichosis is very rare and the etiology is unclear. 4 cases of hemihypertrophy with hemihypertrichosis without any abnormalities, so-called idiopathic hemihypertrophy with hemihypertrichosis, were reported. It needs further evaluation to determine whether this condition is a just incidental coexistence or a new syndrome. Herein, we report a case of idiopathic hemihypertrophy with hemihypertrichosis.