The effectiveness of current treatment recommendations for vitamin D insufficiency in children with CF is unknown. Therefore, we assessed the effectiveness of vitamin D 2 50,000IU once daily for 28days for vitamin D insufficiency.Retrospective chart review of pediatric CF patients from 2006–2008. Vitamin D 2 50,000IU daily for 28days was given to patients with 25-OHD <30ng/mL and repeat 25-OHD levels were obtained after completion of therapy.One hundred forty-seven levels from 97 individuals were assessed. Success of treatment was 54% (n=80/147). Seventeen of 39 patients (43%) followed for an additional 6–18months were able to maintain levels of ≥30ng/mL.Vitamin D 2 50,000IU daily for 28days was effective in correcting vitamin D insufficiency in approximately 50% of subjects. However, almost half of successfully treated patients were unable to maintain normal 25-OHD levels >6months after completion of therapy, implying that this effect is transient.