Myopia is a frequent finding in preterm children. The close association between myopia and retinopathy of prematurity (ROP) suggested a causal relationship, but myopia of prematurity without ROP has yet to be explained. An alternative explanation is presented: myopia is caused by postnatal ellipsoid deformation of the spheric eyeball, i.e. excessive elongation of the optical, as a result of a dolichocephalic deformation of the skull with a short frontal axis. This deformation of the skull is the direct result of bone mineral deficiency frequently observed in very low birth weight infants with increasing postnatal age. This hypothesis is supported by the results of a historical comparison of very low birth weight infants without and with supplementation with calcium and phosphorus that was given in order to prevent postnatal bone mineral deficiency. In the unsupplemented group 4 of 23 children (61%) needed spectacles but only 6 out of 55 (11%) children who were supplemented as babies (questionnaire). Refraction measurements showed myopia stronger than -1 o.d. in group 1 in 5 of 14 infants, whereas in group 2 only 2 out of 22 infants were myopic (P < 0.05).