The authors analyze understanding of humor from the standpoint of its fullness and procedural characteristics in patients with mental illness (N = 31) suffering from: 1) schizophrenia and schizophrenic disorders (N = 19), 2) affective disorders (N = 12) compared to healthy people (N = 39). A set of seven verbal jokes was presented to the subjects followed by an individual semi-structured in-depth interview aimed at discovering levels of cognitive and emotional understanding of humor. Appraisal of the protocols was conducted by an expert group (N = 4) according to five levels of cognitive and three levels of emotional understanding of humor. The findings show a satisfactory level of consistency of expert assessment in both groups. It was confirmed that mental patients are generally characterized by a lower level of understanding humor than the control group both in its cognitive and emotional aspects. Understanding of humor in mental patients differs from that of healthy people both quantitavety and qualitatively; different groups describe different jokes as easy and difficult for understanding: in mental patients the link between cognitive and emotional components of understanding humor is weakened, leading to relative disintegration of the complex process of understanding humor. In schizophrenia the malfunction is more expressed than in affective disorders, however, both components are meaningfully disrupted in both groups, which agrees with the idea of common functioning of intellect and affect. Further research will focus on comparative study of understanding humor in patients with schizophrenia and affective disorders.