Forty-six nursery-reared chimpanzee infants (22 females and 24 males) receiving either standard care (n = 29) or responsive care (n = 17) at the Great Ape Nursery at Yerkes participated in this study. Standard care (ST) consisted primarily of peer-rearing, with humans providing essential health-related care. Responsive care (RC) consisted of an additional 4 hr of interaction 5 days a week with human caregivers who were specially trained to enhance species-typical chimpanzee socio-emotional and communicative development. At 9 months, ST and RC chimpanzees were examined with the Bayley Scales for Infant Development to assess their Mental Development Index (MDI). At 12 months, the chimpanzees were assessed with their human caregivers in the Ainsworth Strange Situation Procedure (SSP). In this first study to use the SSP in chimpanzees, nursery-reared chimpanzees exhibited the definite patterns of distress, proximity seeking, and exploration that underpin the SSP for human infants. In ST chimpanzees the attachment classification distribution was similar to that of human infants raised in Greek or Romanian orphanages. RC chimpanzees showed less disorganized attachment to their human caregivers, had a more advanced cognitive development, and displayed less object attachment compared to ST chimpanzees. Responsive care stimulates chimpanzees' cognitive and emotional development, and is an important factor in ameliorating some of the adverse effects of institutional care.