BACKGROUND Niemann-Pick type C disease (NPC) is a rare genetic disease with an extremely heterogeneous clinical presentation. The adult form of this disease is usually expressed with neurological symptoms; however, non-specific psychiatric disorders are often associated. This article presents a retrospective study on a cohort of NPC patients diagnosed in France with the objectives of researching the presence of psychiatric disorders and qualifying these disorders. METHODS After carrying out a research within hospital records, a questionnaire was sent to patients or their relatives and their doctors. RESULTS The cohort was made up of 22 patients. Fifty-two questionnaires were sent to 47 different doctors. We received 67% of answers from the doctors, with 42% of positive answers. The answer rate of the families was 27%. Among the 22 patients, we found the presence of psychiatric disorders in 86% of cases. Seventy-three percent of the patients presented behavior disorders. Among them, 27% exhibited psychomotor excitation or agitation, 23% aggressiveness, 18% intolerance to frustration, 27% sleep disorders and 23% apathy, listlessness and/or clinophilia. Fifty-five percent of patients presented psychotic symptoms, with 45% with delusions and 36% with hallucinations. Seventy-seven percent of patients presented with mood disorders: 36% suffered from depression and 27% from emotional lability or mood swings. Of the 22 patients, a diagnosis of psychiatric disease was made in 50% of cases. The main diagnoses were schizophrenia (27%) and depression (23%). The other diagnoses identified were acute delirium, dysthymia, infantile psychosis, intellectual disharmonic disability and personality disorder. The average age of emergence of the troubles was 17.1 years old for neurological symptoms and 20.9 years old for psychiatric symptoms. The median age was 18 years old for neurological symptoms and 20 years old for psychiatric ones. In 45% of cases, psychiatric occurred after neurological symptoms; in 27%, they occurred before and they were concomitant in 9%. In 50% of cases, psychiatric symptoms existed before the early diagnosis of NPC; in 9%, they occurred after and in 5%, they occurred during the diagnostic process. Fifty-five percent of the patients were followed by a psychiatrist, 50% of patients had been hospitalized at least once in a psychiatric department. Fifty-five percent of the patients received neuroleptics or antipsychotics, 41% received antidepressants, 41% received mood stabilizers or anticonvulsants, 45% received anxiolytics and 23% hypnotics. CONCLUSIONS Whilst a small cohort, the low incidence of NPC (1/120,000 to 1/150,000) and the scale of a single-center study make the findings important. In our cohort, we found psychiatric disorders in most of the patients. The symptoms were varied and non-specific, and mainly found in late-onset forms of the disease. This incidence of disorder is much higher than the literature suggests, generally describing psychiatric disorders in approximately one third of NPC adult form. On the other hand, our results on schizophrenia are consistent with the updated recommendations for the diagnosis and management of NPC. According to our results, a retrospective study to develop a suspicion index to aid diagnosis of NPC suggests that psychotic symptoms are underestimated in this disease. In our cohort, we also found a significant rate of psychiatric hospitalizations and psychotropic drugs prescription that had not been previously described in the international literature. We did not have sufficient data on the effectiveness of symptomatic treatment in NPC; the literature was contradictory. It should be noted that despite the high rate of psychiatric patients in our cohort, only half of patients consulted a psychiatrist and a few of them have regular follow-up. To conclude, our study is in line with the current literature that suggests an under-estimation of psychiatric disorders in NPC, but also a likely under-diagnosis of NPC in psychiatric departments. All this data encourage us to keep alerting psychiatrists to identifying this disease in order to promote early and optimal care.