Sleep fragmentation in infancy can burden a family by disrupting the sleep of all its members. However, there has been no longitudinal prospective investigation of the determinants of infant sleep fragmentation. We undertook such an investigation. New mothers (N = 106) completed questionnaires and were administered structured telephone interviews at three, six and 12 weeks postpartum. Pre- and postnatal maternal adjustment and sleep-related parental practices were evaluated as potential predictors of infants' sleep fragmentation. Risk factors for infants sleeping less than six consecutive hours per night at 12 weeks were found to be mother's depressed mood (OR = 1.55, p less than 0.01), breastfeeding (OR = 6.40, p less than 0.01) and room sharing (OR = 2.91, p less than 0.05). The mother's depressed mood and breastfeeding were also related to poor sleep consolidation when the latter was assessed as a continuous outcome (p less than 0.01). This study identifies factors to target in sleep-focused interventions in families where the child's sleep has become a problem. (Contains 1 figure and 3 tables.)