Background: Pilomatrixoma is a benign tumor of the skin, preferentially found in children. The final diagnosis is made by biopsy. High resolution ultrasound (US) is a non invasive method for its diagnosis. Aim: To describe the US findings in children with pilomatrixoma. Material and methods: Fifty five patients with 62 clinically suspected pilomatrixomas were studied by US. All examinations were done with an ATL HDI 5000, linear 5-12 MHz transducer. Pathological study confirmed the diagnosis pilomatrixoma in 52 cases. Results: Fifty of 52 pilomatrixomas were diagnosed by US, with a sensitivity of 96%. The mean age of patients was 7.5 years. Forty seven lesions (90%) were located in the head, neck or upper extremities and their mean size was 8.5 mm. Thirty two lesions were hypodermal, 14 were dermohypodermal and 6 were dermal. In 44 lesions the contour was regular and non delineated, 44 lesions were oval, 41 lesions had an acoustic shadow, 36 were echogenic or hyperechogenic, 31 had a peripheral halo (60%), 55 had calcifications (98%), nine had perilesional vessels, 2 had intratumoral vessels and 7 had inflammatory changes. US excluded the diagnosis of pilomatrixoma in 10 lesions formulating a correct differential diagnosis in eight, with a specificity of 80%. Conclusions: Pilomatrixomas had two US types of presentation. The first is a well defined nodule with peripheral halo partially calcified or with microcalcifications. The second is a completely calcified nodule without peripheral halo and with a strong acoustic shadow. US is a useful, sensitive and specific diagnostic method for pilomatrixoma (Rev Méd Chile 2003; 131: 735-40)