Adult thyroid cancers express IGF and IGF-I receptor (IGF-I-R), but the clinical impact is not clear. No previous study examined any childhood thyroid cancers that are well-differentiated and have a favorable prognosis. We used immunohistochemistry to determine IGF-I and IGF-I-R in 23 papillary thyroid cancers (PTC) and 6 follicular thyroid cancers (FTC) from children and adolescents. IGF-I was detected in 45% and IGF-I-R in 43% of cancers. IGF-I and IGF-I-R were found more often in PTC (IGF-I = 9/23, IGF-I-R = 8/19) than normal surrounding thyroid (IGF-I = 0/10, p = 0.032 and IGF-I-R = 0/10, p = 0.030). There were too few FTC to support independent statistical analysis, but IGF-I was found in 4 of 6 FTC (0/10 normal), and IGF-I-R was found in 2 of 4 FTC (0/10 normal). IGF-I-R staining was more intense in aggressive (invasive, metastatic, recurrent, or persistent) than indolent tumors (confined to the gland, p = 0.029). Over time, six tumors recurred, five of which expressed IGF-I-R. Overall recurrence risk was significantly greater for tumors that expressed IGF-I-R (p = 0.05) but only approached statistical significance (p = 0.08) when disease-free survival was determined. We conclude that differentiated thyroid cancers of children and adolescents express IGF-I and IGF-I-R. Tumors that express IGF-I-R are more likely to show aggressive clinical features (invasion beyond the capsule, metastasis, or recurrence) and persistence despite treatment.