Rapid hormonal replacement of children with severe primary hypothyroidism frequently results in irritability and poor concentration. To alleviate these problems we have been using initial low-dose thyroxine treatment, building up to a final dose in an incremental manner over 4 1/2 to 6 months. Because of concern this regimen may compromise growth, we reviewed our experience treating 14 children and adolescents. For the 10 patients with remaining growth potential, 5 to 7 month growth velocity from the onset of treatment was 8.5 +/- 1.9 cm/year (range 5.7-10.9), and 5 to 7 month growth velocity z-score 1.5 +/- 1.7 (range 0.2-4.9). For the entire group, the thyroxine dose required to normalize TSH was 1.6 +/- 0.74 microgram/kg (range 0.9-3.4) or 60.7 +/- 18.9 micrograms/m2 (range 37.5-97.7). Based on the 5 to 7 month z-score, we conclude that satisfactory growth can be achieved on this regimen despite biochemical hypothyroidism. Thyroxine doses required to induce initial euthyroidism are lower than previously proposed.