Background: phenylketonuria (PKU), cause severe intellectual disability in children if neglected 
and can lead to utmost predominant effects of mental delay. Dietary compliance depends on 
people caring for PKU children through modifying their behaviour and knowledge. Aim: this 
study aims to evaluate the dietary adherence regimen program for mothers of children with 
phenylketonuria based on behavior cognitive learning theory. Research design: A quasiexperimental design was used in this study. Settings: The study was conducted at the nutrition 
clinic and Specific Hereditary Outpatient Clinic in Children's Hospital associated to Mansoura 
University Hospital at Mansoura city, Egypt. Subjects and method: the sample comprised of 
100 mothers of children with PKU. Purposive sample technique was used to recruit study 
participants. Tools: The researchers used one tool names structured interview sheet: that contain 
four parts. Part I: demographic characteristic, part II: child medical history, part III: Mothers' 
knowledge, part IV: grounded on Behavioral cognitive learning theory (pre and post). Results: 
this study displayed statistically significant enhancement in mother's knowledge, behaviour 
cognitive learning theory items (22.21 ± 6.83 to 26.90 ± 7.19) (60.27±36.67 to 101.02±40.78), 
respectively pre VS post program. The researchers concluded that there were improving in 
mothers' knowledge, attitude, beliefs, subjective norms, and self-efficacy regarding dietary 
adherence regimen program of their phenylketonuria children based on behaviour cognitive 
learning theory. Recommendations: continuity of dietary adherence regimen program for 
mothers of their children affected with phenylketonuria based on behaviour cognitive learning 
theory ongoing dietetic input is needed to further enhance knowledge of the PKU diet.