On the basis of extensive home observations, Ainsworth proposed that a mother's sensitivity to her infant's signals is the primary determinant of attachment security. Although subsequent research has found a relationship between sensitivity and attachment security, the effect sizes are much smaller than those reported by Ainsworth. In addition to the amount of observation time that might account for the effect size difference, we consider Ainsworth's focus on understanding the organizational structure of relationships. We coded 30 minute video records of interactions between 64 mother-infant dyads from semi-structured home observations conducted at 10 months of age. Coding consisted of writing a narrative summary of the interactions, annotating a completion of Ainsworth's rating scales of acceptance, accessibility, cooperation and sensitivity and then describing the mother's behavior using the Maternal Behaviour Q-set. Sensitivity scores derived from the Q-sort descriptions were robustly related (r = .65) to secure-insecure classifications in the Strange Situation conducted at 13 months. We reflect on the process of assessing Ainsworth's construct of sensitivity.