Face-to-face, dyadic interaction in term and preterm mother-newborn pairs was examined for a rhythmic pattern at 40 weeks gestational age. Three-minute interaction episodes for each mother-infant pair were recorded on videotape and scored with a dyadic code graded along a dimension of disengagement-engagement. Spectral analysis was used to analyze the data for rhythmicity. A reliable rhythmic structure was found in all dyadic interactions. No significant difference was evident between term and preterm dyads. The findings provide quantitative evidence of rhythmic patterning in dyadic interaction from birth for term pairs and as early as 40 weeks gestation for preterm pairs, which is much earlier than previously reported in the literature. The Dyadic Interaction Code, a new tool for studying the interaction process over time, is described, and a recently reported method of analysis (spectral analysis) for interaction data is applied to mother-newborn behavioral data.