Laboratory observations of fathers and infants were made for 24 healthy infants, 23 infants with cystic fibrosis (CF) and 31 with congenital heart disease (CHD) to study possible effects of child chronic illness on father-infant relationships. In addition, both parents completed the Parenting Stress Index on joining the study, at the time of the observation, and a year later. Although the three groups did not differ in reported father stress at any time period or in father-infant interactions, mothers of medically compromised infants reported higher stress at the first two assessments. These findings are consistent with clinical impressions that mothers bear a larger share of the psychological burden of a child's illness than fathers. In spite of the lack of group differences, there was modest evidence within the CF group that fathers who were more involved in treatment were more positively interactive with their infants. Regardless of medical status, higher initial stress reported by both parents and concurrent stress for fathers was associated with less positive father behavior toward the infant. To our knowledge, these are the first observational data concerning father-infant interactions in these two clinical groups. They suggest that differences in father behavior are more likely to be related to the manner in which the family manages the child's illness than the illness per se.