Thyroid hemiagenesis is a rare anomaly with an uncertain incidence, since some patients have been found in a euthyroid state without abnormalities. Females account for 75% and males account for 25% of the cases, giving a female to male ratio of 3:1. The left lobe is absent in 80% of cases and the right lobe is absent in 20% of cases (a left to right hemiagenesis ratio of 4:1). The isthmus is absent in 50% of the patients. A variety of pathological conditions have been found in the remaining thyroid tissue in association with thyroid hemiagenesis: hyperthyroidism, multinodular goiter, hypothyroidism, benign adenoma, adenocarcinoma, Grave's disease. The differential diagnosis would include unilateral inflammatory disease and infiltrative disease, such as amyloidosis. We present two cases diagnosed in pediatric age, with absence of the left lobe and of the isthmus. Thyroid hemiagenesis has been found in association with congenital hypothyroidism in one case. The other patient was found to be in a euthyroid state without abnormalities. The diagnosis of thyroid hemiagenesis should be considered in any patients where, on physical examination, no apparent thyroid tissue is noted on one side of the neck. Sonography demonstrates an absent lobe, an eventually absent isthmus, and an eventually coexiting pathological condition in the remaining thyroid tissue. Sonography may be useful in distinguish between unilateral thyroiditis or other infiltrative processes and true hemiagenesis. Scintigraphy shows the remaining thyroid tissue uptake, and may reveal ectopic thyroid tissue not seen by ultrasound. Ultrasound may be useful in monitoring asymptomatic patients according to the high frequency of pathological condition associated with thyroid hemiagenesis.