A questionnaire to study defects in far vision in the Finnish Twin Cohort Study was sent to 600 twin pairs of 30 and 31 years of age divided equally by gender and zygosity. An excess in female prevalence of defects in far vision was observed (p less than 0.001). Validation of the questionnaire method to assess negative spherical equivalent of refraction was performed by asking the twins to send their last prescription for glasses. Accuracy of the questionnaire method to detect in far vision was tested using a subsample of cases where actual refraction obtained from the prescription for glasses was available. This actual refraction value was compared with the individuals answers to the questions of far vision. Of the subjects 5.5% were false negatives and 1.8% of the individuals were false positives. In 152 pairs one or both twins had a defect in far vision. Under a threshold liability model, the proportion of total (phenotypic) variance in liability attributable to additive genetic effects was 0.91 in this twin data, while no evidence for effects due to dominance was found.