This study examined early and long-term effects of maternal postpartum depression on cognitive, language, and motor development in infants of clinically depressed mothers. Participants were 83 mothers and their full-term born children from the urban region of Copenhagen, Denmark. Of this group, 28 mothers were diagnosed with postnatal depression three to four months postpartum in a diagnostic interview. Cognitive, language, and motor development was assessed with the Bayley Scales of Infant and Toddler Development third edition, when the infants were 4 and 13 months of age. We found that maternal postpartum depression was associated with poorer cognitive development at infant age four months, the effect size being large (Cohen's d = 0.8) and with similar effects for boys and girls. At 13 months of age infants of clinical mothers did not differ from infants of non-clinical mothers. At this time most (79%) of the clinical mothers were no longer, or not again, depressed. These results may indicate 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 absence of other risk factors, this effect may not be enduring. The main weaknesses of the study include the relatively small sample size and that depression scores were only available for 35 of the non-clinical mothers at 13 months.