This is a Phase III, multicenter, open-label study, which will quantify the improvement of nutrient absorption when participants are receiving Ultrase® MT20. The improvement will be demonstrated by comparing the CFA percent (%) and CNA% obtained during a washout of enzyme with the CFA% and CNA% obtained during a period of treatment with Ultrase® MT20. The study is also designed to obtain safety data in CF children suffering also from PI taking Ultrase® MT20. The total duration for the participation of children in this study will be approximately up to 38 days and will include 3 phases: screening phase, the washout phase and treatment phase.