The purpose of this study was to investigate parents' perceptions of their first-born only child's behavior in response to the birth of a second-born sibling. The following major areas were identified for study both before and after the birth of the newborn: (a) parental perceptions of sibling behaviors; (b) parental perceptions of newborn behaviors; and (c) parental perceptions of self behaviors;Subjects were 70 parent couples who were expecting the birth of their second child. The siblings, 32 girls and 38 boys, were 15 to 74 months of age. Questionnaires were completed by the couples during the last trimester of pregnancy and when the newborn was approximately four weeks old;Frequencies and percentages of close-ended items on the two questionnaire sets were calculated. Open-ended responses were grouped. A correlation matrix of all variables was generated. A difference score was calculated for four of the inventories and correlated with other variables. Scattergrams illustrated the responses between mothers and fathers;Significant correlations were found between parents' responses and sibling age and between several factors of parental behaviors and perceived behaviors of the sibling. Parents' expectations and observations of newborn behaviors were related also. Sibling class attendance and perceived behaviors of the sibling after the birth of the newborn were not related. There was a relationship between sibling class attendance and several parental behaviors;The scattergrams indicated strong agreement between mothers and fathers, with generally higher expectations of sibling behaviors before the birth of the newborn than observations after the birth. Mothers showed no pattern for expectations and observations of newborn behaviors, while fathers had higher expectations. Higher parental expectations for newborns correlated positively with desirable newborn behaviors. Parents self-expectations were much higher than self-observations. Three areas of positive sibling behaviors and five areas of negative sibling behaviors were reported in the open-ended questions. This study indicates many variables for further study of the sibling relationship and parent-sibling-newborn interactions.