Mothers in prison represent a high-risk parenting population. New Beginnings is an attachment-based group intervention designed specifically for mothers and babies in prison. This cluster randomized trial examined the outcomes for 88 mothers and babies participating in the New Beginnings program and 75 dyads residing in prisons where the intervention did not take place. Outcomes were measured in terms of parental reflective functioning, the quality of parent-infant interaction, maternal depression, and maternal representations. Mothers in the control group deteriorated in their level of reflective functioning and behavioral interaction with their babies over time, whereas the mothers in the intervention group did not. There were no significant group effects on levels of maternal depression or mothers' self-reported representations of their babies over time. An attachment-based intervention may mitigate some of the risks to the quality of the parent-infant relationship for these dyads.