Oxidative stress often affects the structure and metabolism of lipids, which in the case of polyunsaturated free fatty acids (PUFAs) leads to a self-catalysed chain reaction of lipid peroxidation (LPO). The LPO of PUFAs leads to the formation of various aldehydes, such as malondialdehyde, 4-hydroxynonenal (4-HNE), 4-hydroxyhexenal, and 4-oxo-2-nonenal. Among the reactive aldehydes, 4-HNE is the major bioactive product of LPO, which has a high affinity for binding to proteins. This review briefly discusses the available information on the applicability of assessment options for 4-HNE and its protein adducts determined by immunosorbent assay (the 4-HNE-ELISA) in patients with various diseases known to be associated with oxidative stress, LPO, and 4-HNE. Despite the differences in the protocols applied and the antibodies used, all studies confirmed the usefulness of the 4-HNE-ELISA for research purposes. Since different protocols and the antibodies used could give different values when applied to the same samples, the 4-HNE-ELISA should be combined with other complementary analytical methods to allow comparisons between the values obtained in patients and in healthy individuals. Despite large variations, the studies reviewed in this paper have mostly shown significantly increased levels of 4-HNE-protein adducts in the samples obtained from patients when compared to healthy individuals. As with any other biomarker studied in patients, it is preferred to perform not only a single-time analysis but measurements at multiple time points to monitor the dynamics of the occurrence of oxidative stress and the systemic response to the disease causing it. This is especially important for acute diseases, as individual levels of 4-HNE-protein adducts in blood can fluctuate more than threefold within a few days depending on the state of health, as was shown for the COVID-19 patients.