Studies of parent-child interactions during perinatal addiction treatment are needed to offer insights into the nature of child development outcomes for this population. A purposeful sample of 17 women in a drug and alcohol addiction treatment program and their infants was studied over the course of their first year in the program. The purpose of this naturalistic field study was to examine parent-child interactions during addiction treatment. The following research question guided the inquiry: What is the nature of parent-infant interaction during the first year in a perinatal addiction treatment program? This study combined grounded theory methodology with the quantitative methods of descriptive and differential statistics. Open-coding of interviews, field notes, and the treatment records followed grounded theory methodology. A time-sampling procedure was used to examine stability of interactions at the beginning of program participation, again when each infant was 6 months old, and when each infant was 12 months. Results are used to describe parent-child interactions across samples and over time.Chi-square procedures were performed on number of children in each family, length of time in the program, sobriety, and mutual enjoyment of interaction. A significant difference in interaction by length of time in the program (χ2= 7.0801, df = 1, p<. 00) as well as in interaction by percent sobriety (χ2= 4.1538, df = 1, p<. 04) was evident. Behaviors of the infants reflected the emotional tone and parenting responses of their mothers. Dyads whose interactions showed a pattern of mutuality were more likely to continue in the treatment program and to maintain sobriety. The ability of a mother to enjoy her infant seemed to be the one factor that made the most difference in child behaviors. Results can be used by clinicians to guide the choice of treatment approaches that support the parent-child relationship during addiction treatment.