Objective To determine the prevalence and nature of developmental delay at preschool age in infants born moderately preterm compared with those born full-term and early preterm. Study design Parents of 927 moderate preterm infants (32-35+6 weeks gestation), 512 early preterm infants (<32 weeks gestation) and 544 full-term infants (38-41+6 weeks gestation) completed the Ages and Stages Questionnaire (ASQ) when the child was aged 43-49 months. We analyzed rates of abnormal ASQ scores and odds ratios for abnormal ASQ scores in both preterm groups compared with the full-term group. We repeated the analyses after adjustment for socioeconomic status, sex, being part of a multiple birth, and small for gestational age status. Results Abnormal (ie, >2 SDs below the mean) ASQ total scores were noted in 8.3% of moderate preterm infants, in 4.2% of full-term infants, and in 14.9% of early preterm infants. ORs of abnormal ASQ total scores were 2.1 (95% CI, 1.3-3.4) for moderate preterm infants and 4.0 (95% CI, 2.4-6.5) for early preterm infants. Both moderate and early preterm infants had more frequent problems with fine motor, communication, and personal-social functioning compared with full-term infants. Compared with full-term infants, moderate preterm infants did not have a greater prevalence of problems with gross motor functioning and problem solving, whereas early preterms did. Socioeconomic status, small for gestational age status, and sex were associated with abnormal ASQ scores in moderate preterm infants. Conclusions At preschool age, the prevalence of developmental delay in moderate preterm infants was 2-fold of that in full-term infants and one-half of that in early preterm infants.