Introduction: Cystic fibrosis (CF) is an autosomal recessive disease caused by a mutation in the CFTR gene, resulting in an alteration of a protein involved in sodium and chloride transport in the apical plasma membrane of epithelial cells in respiratory and intestinal tracts. It primarily presents respiratory compromise, affecting other systems in different ways. Meconium ileus is a gastrointestinal manifestation that occurs in 10-20% of patients, which is not entirely attributable to a specific CFTR mutation. Objective: To report a case of monozygotic twins diagnosed with CF (F508) in whom phenotypic variation is evident based on the expression of meconium ileus, showing that there are external modifiers in the development of this complication. Case report: monoamniotic monochorionic twin pregnancy which resulted in preterm births. One of the patient presented meconium ileus at birth leading to CF suspicion and establishing the diagnosis by (F508/F508) molecular analysis in both twins. Conclusion: Phenotypic variability in these twins supports the hypothesis proposed by different authors that there are other gene expression-modulation factors of the disease as well as environmental modifiers that must be taken into account when dealing with this disease.