Sensitivity hypothesis has been corroborated in different contexts and children ages; however, there is lack of studies that have analyzed it in the first year of life. The present study related maternal sensitivity and child secure base behavior in a group of 32 Peruvian mothers of low income families. Mothers were 19 to 44 years old and their children were between 8 and 10 months old. Mother's sensitivity was assessed using the Q-sort Maternal Behavior (Pederson & Moran, 1995) and child secure base behavior through the Attachment Q-sort (Waters, 1995). Results indicated a significant and direct relationship between two constructs and that early mother-infant physical separations play an important role in this relationship.