Fifty-five patients (38 and 17), whose age ranged 14 to 153 days, with a clinical suspicion of sternocleidomastoid mass, were examined with US. Neck movements were impaired in all patients and 6 of them exhibited no clinically palpable mass. Sternocleidomastoid masses were detected in 50 patients. Other neck masses (lymphadenopathy, neuroblastoma, lymphangioma) were demonstrated in the remaining 5 patients. US sensitivity was confirmed to be higher than that of clinical palpation in detecting the masses. Our results show that males (69%) and the right side (80%) were mostly affected. The sternal head (92%) and the lower third of the muscle (88%) were most commonly involved. Forty-four masses (88%) had homogeneous echostructure and their echogenicity relative to normal muscle was mostly isoechoic in 33 patients (66%) and hyperechoic in 11 patients (22%). Six masses (12%) had a patchy echotexture and mixed echogenicity. No cases of mostly hypoechoic echotexture or intralesional calcifications were observed. The mass to muscle ratio in the largest transverse sections ranged 25-175% (mean: 95%). Sternocleidomastoid masses disappeared in all 50 patients within their 6th month of age. US is confirmed as a valuable tool to diagnose and follow-up sternocleidomastoid masses.