A 3½ year old white girl attended the paediatric eye clinic. Her mother had noticed she had been screwing her eyes up and blinking excessively. This occurred mainly when she was reading and towards the end of the day, and it had worsened over six months. The child was otherwise well, was taking no drugs, and had been born at full term by normal delivery at a weight of 3200 g. She had not had any previous eye treatment. Her mother and maternal grandfather had had strabismus from early childhood. Testing revealed a constant slight right convergent strabismus for near and distance fixation (near 12 prism dioptres; distance 8 prism dioptres). Unaided vision of the right and left eyes was 6/30 (0.7) and 6/7.5 (0.1). Depth perception was negative. Ophthalmoscopy was normal and cycloplegic refraction revealed bilateral moderate hypermetropia (long sightedness) with significant anisometropia (difference in refractive error between the two eyes; +5.00 dioptre spheres (DS) in the right eye and +3.50 (DS) in the left eye).