Background: The Newborn Hypotonic Syndrome (NHS) is a clinical entity that presents up to 28 days after birth. The main symptom is a significant decrease in muscular tone, but its severity is determined by the lack of muscular strength. NHS is a relatively frequent entity, so it becomes an important diagnosis problem. There is few information in literature regarding its incidence. Method: Retrospective study performed between May 2000 - April 2006, including patients with diagnosis of NHS in a Neonatal Intesive Care Unit. Results: 2 158 newborns, of which 113 (5.2%) had NHS. 83% of cases were attributed to central causes, such as hipoxic-isquemic encephalopathy (49%) and genetic disorders (15%). 17% of cases corresponded to peripheral causes, including hypermagnesemia (68%) and myopathic diseases (21%). Conclusions: 1) NHS is a relatively frequent clinical entity; 2) Central causes are the most prevalent; 3) It is possible to study the etiology of NHS with a systematic approach