Evidence-based treatments that achieve optimal energy intake and improve growth in preschool-aged children with cystic fibrosis (CF) are a critical need. To test whether behavioral and nutritional treatment (intervention) was superior to an education and attention control treatment in increasing energy intake, weight z (WAZ) score, and height z (HAZ) score. This randomized clinical trial included 78 children aged 2 to 6 years (mean age, 3.8 years) with CF and pancreatic insufficiency (intervention, n = 36 and control, n = 42). The study was conducted at 7 CF centers between January 2006 and November 2012; all 78 participants who met intent-to-treat criteria completed through follow-up. Behavioral intervention combined individualized nutritional counseling targeting increased energy intake and training in behavioral child management skills. The control arm provided education and served as a behavioral placebo controlling for attention and contact frequency. Both treatments were delivered in person or telehealth (via telephone). Sessions occurred weekly for 8 weeks then monthly for 4 months (6 months). Participants then returned to standard care for 1 year, with 12-month follow-up thereafter. Changes in energy intake and WAZ score were examined from pretreatment to posttreatment (6 months) and change in HAZ score was assessed pretreatment to follow-up (18 months). Covariates included sex, Pseudomonas aeruginosa status at baseline, and treatment modality (in person vs telehealth). At baseline, mean (SD) energy intake was 1462 (329) kcals/d, WAZ score was -0.44 (0.81), and HAZ score was -0.55 (0.84). From pretreatment to posttreatment, the intervention increased daily energy intake by 485 calories vs 58 calories for the control group (adjusted difference, 431 calories; 95% CI, 282 to 581; P < .001) and increased the WAZ score by 0.12 units vs 0.06 for the control (adjusted difference, 0.09; 95% CI, -0.06 to 0.24; P = .25). From pretreatment to follow-up, the intervention increased the HAZ score by 0.09 units vs -0.02 for the control (adjusted difference, 0.14 units; 95% CI, 0.001 to 0.27; P = .049). Measured treatment integrity and credibility were high for both groups. Behavioral and nutritional intervention improved energy intake and HAZ score outcomes but not WAZ score outcomes. Our results provide evidence that behavioral and nutritional treatment may be efficacious as a nutritional intervention for preschoolers aged 2 to 6 years with CF and pancreatic insufficiency. clinicaltrials.gov Identifier:NCT00241969.