Alterations in the relative proportions of very low density lipoprotein (VLDL) apoprotein C-III (apoC-III) isoforms have been previously observed under various dietary and metabolic states. This study was designed to evaluate the effects of a very high caloric restriction on plasma triglycerides and on the relative proportions of VLDL apoprotein C-II (apoC-II) and apoC-III isoforms in obese subjects. VLDL were isolated by preparative ultra-centrifugation from 12 obese women. ApoC-II and apoC-III subspecies were separated by analytical ultrathin-layer isoelectrofocusing. The mean body weight decreased significantly from 96.2 ± 8.7 to 90.6 ± 7.6 kg (p < 0.01). Mean total and VLDL triglycerides did not vary significantly. Apoprotein C-III2 (apoC-III1) as percentage of total apoprotein C increased (p < 0.01) and apoprotein CIII0 (apoC-III0) decreased (p < 0.01). Apoprotein C-III1 (apoC-III1) did not vary significantly. An inverse correlation was found between the percentage variation of apoC-III2 and that of apoC-III1 (r = -0.94; p < 0.01). The variations of apoC-III2 correlated positively (r = 0.83; p < 0.01), while those of apoC-III1 correlated inversely (r = -0.65, p < 0.025) with the changes of VLDL triglycerides. The apoC-III1 to apoC-III2 ratio as well as the apoC-III0 to apoC-III2 and apo C-III1 ratios decreased after diet (p from < 0.01 to < 0.001). Total apoC-III as well as apoC-III2 and apoC-III1 to apoC-II ratios did not vary. On the contrary, the apoC-III0 to apoC-II ratio decreased (p < 0.001). The data indicate that severe caloric restriction is associated with a modification in the relative proportions of VLDL apoC-III subspecies. Further studies are needed to define if these changes play a role in regulation of plasma triglycerides in ma