Prader–Willi syndrome (PWS) is a neurodevelopmental disorder caused by a genomic imprinting with deficiency of paternally expressed gene or genes in the chromosome 15q11-13 region. Its estimated prevalence is 1:10,000 to 1:30,000. The clinical characteristics of the syndrome are infantile hypotonia, short stature, characteristic facial features, intellectual disabilities, hyperphagia and increased risk for obesity. Psychiatric disturbances such as cognitive impairment and maladaptive behaviors including hyperphagia and non-food compulsive behaviors are well described in PWS patients. Also according to the recent knowledge, there is an increased risk of affective and psychotic disorders. In this report, we aimed to present a patient with a delayed diagnosis of a Prader Willi syndrome and to note the importance of early detection. A 41-year-old female applied to our emergency clinic with suicidal ideation, impulsivity and aggressive behaviors. In her mental examination, borderline intellectual functioning was predicted and in her physical appearance central obesity, short stature, specific facial features was observed. During inpatient follow up, genetic consultation revealed “abnormal methylation in SNRPN gene” consistent with Prader Willi syndrome. In this case, not only delayed diagnosis of the syndrome but also the medications prescribed so far may aggravate the core clinical symptoms such as obesity and behavioral problems.