Differentiated thyroid cancer (DTC), including papillary (PTC) and follicular (FTC) variants, is unusual in children and accounts for only 10% of all cases. For that reason, knowledge of the clinical features which predict recurrence is limited. We reviewed 170 cases of childhood DTC to determine if specific clinical or pathological findings were associated with increased risk of recurrence. This was a retrospective study of children and adolescents with DTC registered in the Department of Defense Automated Centralized Tumor Registry. We reviewed 137 cases of PTC and 33 cases of FTC diagnosed between 1953 and 1996 at < or = 21 years of age. In the PTC group (median follow-up 6.6 years, range 2 month-39.5 years), only one patient died, but 21 developed local and 6 developed distant recurrence. By univariate analysis, recurrence was more common in patients with multifocal (odds ratio 7.5) or large tumours (odds ratio 4.1), and in those with palpable cervical lymphadenopathy (odds ratio 3.0) or metastasis at diagnosis (odds ratio 2.8). By multivariate analysis focality was the best predictor of recurrence (P = 0.0019). In the FTC group (median follow-up 5 years, range 6 month-38.1 years), no patient died of disease, but 5 developed recurrence. As with PTC, recurrence was more likely in patients with multifocal tumours (odds ratio 22.0). Differentiated thyroid cancer in children and adolescents has low mortality, but a high risk of recurrence. Young patients with large, multifocal tumours that are already metastatic at diagnosis have the greatest risk of recurrence.