Resonance Raman spectra of low-density lipoprotein (LDL) isolated from the plasma of diabetic patients (age range 13-77 yr, mean age 37 yr) and age- and sex-matched control subjects were recorded in the 1000- to 1600-cm-1 region as a function of temperature (0-50 degrees C). Both nondiabetic and diabetic LDL yield spectra characterized by two major bands near 1160 and 1530 cm-1 due to the carotenoid component of lipoproteins. The relative intensity of 1530- and 1160-cm-1 bands, assigned to -C = C- and = C-C = stretchings, respectively, i.e., I1530-I1160 ratio, was plotted against temperature. For nondiabetic control subjects, the plots showed an inflection in the temperature range of 30-39 degrees C, which corresponded to the thermal transition of LDL. This transition was abolished in the LDL of diabetic patients (P less than 0.001), suggesting an altered lipid structure. The transition (30-39 degrees C) was also abolished in the in vitro glycosylated nondiabetic LDL. Lipid analysis did not show any appreciable change between nondiabetic control subjects and diabetic patients. The change in the thermal transition properties of diabetic LDL has been attributed to the organizational change in the LDL protein.