Background: It is well documented that maternal postpartum depression (PPD) has the potential to disrupt aspects of caregiving known to be critical for healthy child development. However, with regard to long term effects of PPD on global indices of infant development measured by standardized instruments, findings across studies have been inconsistent. Some studies have found small, but significant effects, and other studies found no long term effects, even for vulnerable subgroups. Given that PPD onset for the majority is within 3 months postpartum, and given that most women recover in 6-7 months postpartum, it is possible that potential adverse effects of PPD on infant development for a large part have diminished or buffered by protective factors at the time when infant development is measured. However, little is known about how the concurrent exposure to maternal depressed mood impacts on infant development before 6 months of age, that is, at the time when maternal postpartum depressive symptoms are most present. To address this issue, additionally to a long-term measure of infant development, the present study examines early concurrent effects of maternal clinical depression. Method: Mothers (N=85) were recruited into a longitudinal study and assessed with Edinburgh Postnatal Depression Scale and the clinical interviews Present State Examination and Structured Clinical Interview for DSM-IV Axis II. The Bayley Scales of Infant and Toddler Development, third edition, were administered when infants were 4 and 13 months of age. Results: MANCOVA revealed a significant adverse effect of maternal depression on infant cognitive development at four months of age, the effect size being large, and with similar effects for boys and girls. At 13 months of age infants of mothers who had been suffering from depression three to four months postpartum did not differ from infants of non-clinical mothers. However, regardless of depression group membership boys scored lower on the language scale at 13 months of age. Discussion: These results suggest that maternal depression can have an acute, concurrent effect on infant cognitive development as early as at four months postpartum; at the same time, in the lack of other risk factors, this effect may not be enduring. From a developmental psychopathology perspective this study stresses the importance of understanding the complex nature of how risk factors may impact on infant development differently at different ages. Instead of pointing selectively to a single predictor of developmental outcome, such as PPD, rather a combination of several risk factors may predict children’s long-term developmental problems.