Smith–Magenis syndrome is a syndrome of congenital malformations, classified as rare diseases. It appears in one child in 15,000–25,000 births. The disorder is caused by deletion of chromosome 17p11.2 or mutations in the RAI gene. The image of the disfunction is heterogeneous and includes numerous neuropsychological deficits. Changes in the chromosomal system lead to phenotypic changes typical for this syndrome, delayed psychomotor development, delayed speech development, intellectual disability, and emotional and behavioural disorders. The Smith–Magenis syndrome also includes serious sleep disorders. This paper presents the clinical case of a girl with Smith–Magenis syndrome. The aim of this study is to increase theoretical and practical clinical knowledge about this disorder, and to understand characteristic features of the child’s functioning based on neurological, neuropsychological and behavioural symptoms.