Over the past years, 24 patients with Phelan-McDermid syndrome were carefully investigated with respect to history, somatic and neurologic antecedents, treatment history, behavioural issues, and psychiatric symptoms including possible catatonic features and regression phenomena. Patients were originally referred for specialized diagnosis and treatment advice because of recurrent challenging behaviours along with instable mood. In all, standardized neuropsychiatric examination was performed including assessment of intellectual and adaptive functioning as well as communication and behaviour concerns. Psychiatric diagnoses were actualized in interdisciplinary consultation meetings according to ICD-10 guidelines. The course of disease was periodically monitored with respect to treatment efficacy and psychopathology over a period varying from one to five years. In 18 patients, a deletion encompassing part of or the entire SHANK3 gene was found. All comprised two or more genes in addition to SHANK3. In six patients, a pathogenic variant in this gene was detected. The psychopathological profile of all patients (nine were published before) was characterized by symptoms from the autism and schizoaffective spectrum while in five, periodic catatonic symptoms were also established. In their third decade, four patients with the deletion subtype developed a regression-like gradual decline of functioning. Based on actual psychiatric classification, in 18 patients, a diagnosis of atypical bipolar disorder was established of which symptoms typically started from late adolescence onward. In most patients, treatment with mood stabilizing agents in combination with individually designed contextual measures, and if indicated with the addition of an atypical antipsychotic, resulted in gradual stabilization of mood and behaviour.