Autism spectrum disorder (ASD) is a heterogeneous group of neurodevelopmental disorders that can affect up to 2.6% of the population. Most of these people will have at least one other psychiatric disorder, often diagnosed with a delay or not recognized at all. This study describes the epidemiology, diagnostic difficulties and potential treatment of patients with ASD and comorbid bipolar disorder (BD). The prevalence of bipolar disorder in ASD is estimated at 5–8%. The study with the most numerous group included 700,000 children out of which 9,062 fulfilled ASD criteria at 16 – BD was found to be 6 times more prevalent in this group compared to the control group. Many factors affect the diagnosis. Patients with ASD often have limited insight into understanding the complex emotional states and difficulty in expressing them due to their impairment. The symptoms of bipolar disorder are in their case unspecific and differ from those occurring in the general population, which makes it difficult to make proper diagnosis. Despite the lack of research on the group of patients with ASD, psychometric tools designed to evaluate the general population are used to examine patients. This work aims to show the current state of knowledge and highlight areas that require further investigation