Background: Cognitive deficits are one of the most common adverse effects of chemotherapy (CHT). Previous reports suggest that this is due to the so-called chemo brain syndrome, the symptoms of which manifest mainly as impairments in executive functions, speed of information processing, memory, attention, and motor speed. However, empirical evidence for these manifestations is currently ambiguous. Methods: The research group consisted of 26 cancer patients with haematological malignancies who had undergone chemotherapy treatment. Cognitive performance was measured by two screening cognitive tests, the Mini-Mental State Examination (MMSE) and the Montreal Cognitive Test (MoCA). Results: MMSE detected cognitive deficits in 34.6% of patients whereas MoCA identified mild or moderate cognitive impairment in up to 80.7% of patients. The highest error rates were found in tasks focused on memory, attention, spatial orientation, executive functions, and abstraction. Cognitive deficit progressed with age, but not with duration of therapy. Conclusion: Deficits in cognitive functions occur in a considerable number of patients after CHT, although the diagnosis depends on the sensitivity of the detection method. Screening scales usually provide the first indication of impaired cognitive functioning and may indicate the need for further neuropsychological examination. Early diagnosis of reduced cognitive functions is a prerequisite for effective psychological intervention to help patients cope with the undesirable effects of chemotherapy treatment more quickly.