Very low birth weight (VLBW; less than 1500 g) and/or very preterm children are at risk for strabismus. However, the age of onset of strabismus is still unknown. The present study reports on the frequency of strabismus in 450 5-year-old children born with a birth weight of less than 1500 g and/or with a gestational age below 32 weeks. The age at which strabismus was initially diagnosed was determined retrospectively. At 5 years of age 65 of the at-risk children (14.4%) presented with strabismus. Fifteen of them (3%) had at that age not been referred to or treated by an ophthalmologist. At 3 years of age only 28 strabismic children were being treated; 7 at-risk children (2%) had been treated for strabismus before the end of the 1st year. These results were compared to those from a second study in which eye alignment was longitudinally examined in 194 VLBW children from 6 weeks until 12 months of (corrected) age and additionally in 65 of these children at the age of 2.5 years. At a first glance, the frequency of strabismus in the longitudinal study seemed rather stable during the first 2.5 years of life, with values varying between 14% and 18%. However, only a small percentage of misalignments which were noted at 6 weeks of age persisted until 2.5 years. On the other hand, if children had a misalignment at 9 months, strabismus was still present when they were reexamined at 1 and at 2.5 years of age.(ABSTRACT TRUNCATED AT 250 WORDS)