Sleep-related rhythmic movement disorder (RMD) is common in very young children but rarely persists beyond childhood. Despite its high frequency, the underlying pathophysiology remains unclear. Familial occurrence is rare. Here we present monozygotic female triplets, all of them being affected by body rolling in terms of RMD. Furthermore, they all present with an additional genetic disease, cystic fibrosis, with the same documented mutation of the cystic fibrosis transmembrane conductance regulator gene (F508del-CFTR). Because all three monozygotic siblings are concordant for RMD, genetic factors may contribute to the time course of the disorder.