Parental report may provide an inexpensive alternative to standardized assessments of children's development. We have adapted and validated a parental questionnaire on cognitive development for use with very preterm infants. Sixty-four 2-year-olds (28 males, 36 females) born less than 30 weeks' gestation (median 28.5 weeks, range 23 to 31 weeks), median birthweight 980g (range 500 to 1905g) were assessed using the Mental Development Index (MDI) of the Bayley Scales of Infant Development-II and the parental questionnaire. Significant correlations between parent report and MDI scores (r=0.54-0.68, p<0.001) indicated good concurrent validity. Diagnostic use of the parent report for predicting an MDI score of less than 70 was assessed by using receiver operating characteristic (ROC) curves. The optimal cut off produced equal sensitivity and specificity (81%), indicating good discriminatory power in diagnosing developmental delay. Test-retest reliability was demonstrated and accuracy of parent reporting was not affected by sociodemographic factors. Our questionnaire provides a valid outcome measure for use in randomized trials or large population surveys.