The incidence of amblyopia was analysed in a group of 20 patients with early onset esotropia. These patients reached adulthood without any form of previous treatment. The incidence of amblyopia was compared in a group of 20 patients who received conventional treatment, including occlusion and early surgical alignment. Only three patients (15%) in the untreated group presented with amblyopia, compared with 16 (80%) in the treated group. After treatment 35% of the control group remained amblyopic. Spherical anisometropia of more than 2 dioptres was present in two of the patients with amblyopia in the untreated group, but was not associated with amblyopia in the control group. Early surgical alignment permits the development of peripheral fusion, allowing long term alignment stability, but amblyopia appears to be more common after surgical alignment.