Constipation is a frequently encountered problem in childhood, with a prevalence ranging between 1 and 30%. Several studies have proposed pelvic ultrasound, (simple, non-invasive and reproducible) both to define the presence of megarectum, and to follow the answer to treatment, but the real utility remains to be defined, especially in the follow-up.The primary aim of the study is to evaluate whether, in the conventional treatment of functional constipation of the child, the variations of the rectal diameter, measured through the use of the pelvic ultrasound, are a good marker of disease severity and efficacy of the treatment.