We assessed parental differential treatment of siblings (maternal time, affection, discipline) in normal and high-risk families. Differential treatment was measured using home interviews, nightly phone ratings, and daily diaries tracking how mothers spent their time. Subjects were 40 mothers of toddlers (average age 2 years) and preschoolers (average age 4 1/2 years): half were caring for a younger child with a chronic illness (i.e., cystic fibrosis), and half were caring for two healthy children. Little evidence of parental differential treatment was found in the home or phone interview data. However, on the diary variables, both quantitative and qualitative differences in parental treatment were found in cystic fibrosis (CF) versus comparison families. Specifically, mothers spent more individual time with younger, chronically ill children in play and mealtime activities than with their older, healthy siblings. Further, mothers in the CF group rated time spent with older children as significantly more negative than time spent with younger children. Convergence between measures of differential treatment and advantages of using a high-risk comparison approach are discussed.