The study of factors that reduce subjective well-being is necessary to improve the quality of a patient's social functioning and the effectiveness of treatment in chronic somatic diseases. The aim of this study was to investigate the relationship between illness perceptions and subjective well-being in patients with diabetes mellitus and with ischemic heart disease. It was found that patients with type 2 diabetes mellitus and with coronary heart disease are more in need of help to improve subjective well-being than patients with type 1 diabetes mellitus. according to the results of the moderation analysis, patients with diabetes, as with chronic diseases in general, have higher satisfaction with physical health with less explicit perceptions of cyclicality, consequences of the disease, its emotional representations, and higher treatment control; life satisfaction and positive experiences are more pronounced with low levels of explicit perceptions of negative consequences of the disease, as well as life satisfaction with higher levels of self-efficacy for treatment; they have higher levels of self-efficacy for treatment; and they have higher levels of physical health satisfaction and positive experiences with higher levels of self-efficacy for treatment. At the same time, patients with type 2 diabetes, unlike patients with type 1 diabetes, have a worse understanding of various aspects of the disease, and their perceptions that the disease is cyclical and unstable are more strongly associated with a worse assessment of their own physical health. In the latter, lower overall life satisfaction was more strongly associated with lower levels of self-efficacy. Based on the results obtained, conclusions were made that psychological work with patients with chronic diseases should be aimed at forming patients' perceptions of the cyclicality and consequences of the disease, treatment control and self-efficacy of the patient in relation to treatment, corresponding to reality, as well as correction of emotional representations. In patients with type 1 diabetes mellitus, the level of self-efficacy is of particular importance in assessing well-being, and in patients with type 2 diabetes mellitus, the degree of understanding of various aspects of the disease, and perceptions of its cyclicality/stability.