OBJECTIVE: Co-morbidities in any disorder can complicate its diagnostic process, they require more complex clinical management and lead to worse health outcomes and increased healthcare costs. There are regional differences in the prevalence of specific co-morbidities in Parkinson's disease (PD), and data from middle Europe are lacking. The project COSMOS aimed to disclose the prevalence of co-morbidities among patients with PD in Slovakia. METHODS: In a national, multi-center, cross-sectional, observational study, neurologists gathered relevant demographical and clinical data aimed at all psychiatric and somatic co-morbidities. RESULTS: From overall 737 patients, 51.00 % had at least one psychiatric co-morbidity, the most prevalent were depressive episode/recurrent depressive disorder (26.05%), sleep disorders (23.20%), dementia (13.16%), and neurotic, stress-related, and somatoform disorders (11.53%). In addition, 92.9 % had at least one somatic co-morbidity, the most prevalent were hypertensive diseases (67.71%), ischemic heart diseases (42.74%), diseases of the musculoskeletal system, and connective tissue (39.21), and disorders of lipoprotein metabolism (33.24%). The number of psychiatric co-morbidities increased with PD progression; the prevalence of somatic comorbidities increased also with the age (p<0.001 in all cases). CONCLUSION: This study with a large cohort of PD patients confirmed a high prevalence of depression, dementia, sleep problems, and anxiety disorders, together with cardiovascular disorders, diseases of the musculoskeletal system, and metabolic syndrome. With PD progression, the number of both psychiatric and somatic co-morbidities is on the increase. If not treated properly, they lead to more complicated management. That's why it's essential to search for any co-morbidity to provide patients with early and adequate therapy to avoid further worsening of quality of life.