The factors, influencing the development of cognitive defects in patients after chemotherapy are the following: personal individual intellectual and genetic characteristics, immune dysfunction and neurotoxicity. Cognitive domains, disturbed to a greater extent, are memory, attention, thinking rate and executive functions. Sixty one patients, diagnosed lymphoproliferative disease, (chronic lympholeukosis – 28 persons, multiple myeloma – 33) were examined; patients’ mean age was 63.87 ± 9.76 years. Reduced cognitive function was revealed in 73.77 % of patients, a mean score by MoCA scale was 22.87 ± 4.08. Cognitive defects negatively influence patients’ quality of life, significantly reduce their physical and mental working ability, everyday activity, make execution of professional duties difficult, besides they can complicate nursing.