AbstractWe report a 3.5‐year‐old Japanese boy who developed lichenoid papules and erythema with noncaseating epithelioid cell granulomas with a scant lymphocytic infiltrate histologically on his limbs at the age of 8 months. Genetic analysis of the patient and his parents, who had no medical past history, revealed heterozygous 1147G>A (E383K) mutation of NOD2 in the patient and in his father, so the patient was diagnosed with Blau syndrome and his father as an asymptomatic carrier. Although Blau syndrome has been reported as a genetic disease with high penetrance, asymptomatic carrier cases of a family with the same E383K mutation have also been reported. These results suggest that some contributing factors are required for the development of inflammatory and granulomatous responses in heterozygous carriers of a NOD2 E383K mutation.