Aims: To examine the relationship between failure to thrive in preterm and term infants and postnatal depression in their mothers. Method: In a whole population birth cohort of 12,391 infants (excluding those born after term or with major congenital abnormalities) failure to thrive over the first nine months was identified using a conditional weight gain criterion which identified the slowest-gaining 5%. Depression symptoms were recorded using the Edinburgh Postnatal Depression Scale (EPDS) at 18 and 32 weeks of pregnancy and at 8 weeks and 8 months after delivery. Results: After the birth, high depression scores were significantly more common in the mothers of infants born preterm, and controlling for depression scores in pregnancy did not eliminate this association after the birth. Failure to thrive was identified in 4.5% of the children born at term (531/11718) and in 8.3% of those born preterm (56/673). The difference was highly significant ([chi][superscript 2] = 20.25 with 1 df, p less than 0.0001). Using a conventional cut-off on the EPDS (score greater than 12) to identify mothers as "depressed", the prevalence of failure to thrive in the term infants of mothers depressed at 8 weeks postpartum was 5.0%; in the remainder of the population (controls) it was 4.3%. In mothers depressed at 8 months the prevalence was 4.3% in both groups. The prevalence of failure to thrive in the preterm infants of mothers depressed at 8 weeks was 8.8% (7.0% in controls) and in those depressed at 8 months it was 12.3% (6.7% in controls). None of these differences in prevalence was statistically significant, and significant differences did not emerge from further analyses using more stringent criteria for depression. Conclusions: Preterm births are specifically associated with high maternal depression scores in the postpartum period, and with a higher prevalence of failure to thrive. High depression scores in the postpartum period are not themselves associated with a higher prevalence of failure to thrive, however, either in infants born at term or in those born preterm.