Side-effects following long-term endocrine therapy might have clinical implications. The aim of this study was to study potential methods to detect effects on plasma induced by hormonal therapies. The composite methylene (chemical shift between 1.2-1.4 ppm) and methyl (0.8-0.9 ppm) aliphatic peaks of the 1H magnetic resonance spectrum (500 MHz) were analysed in consecutive plasma samples of 23 cancer patients drawn before and during treatment with hormonally acting drugs. The aliphatic peaks were analyzed for line width at half-height and then averaged. In addition, 13C magnetic resonance spectroscopy (125 MHz) analyses were done in selected patients. The blood samples were analyzed for triglyceride, cholesterol, apolipoprotein A1 (apo A1), and apolipoprotein B (apo B) levels. The methylene line width increased significantly after 9 weeks of tamoxifen (41.4 vs. 37.6 Hz). A trend of differences was observed in the saturated part of the 13C magnetic resonance spectrum. A significant decrease in total cholesterol (mean decrease, 13%), increases in apo A1 (9%) and in the ratio of apo A1 to apo B (28%), but unchanged total triglycerides were found, indicating a decrease in LDL and increase in HDL lipoproteins in these patients following tamoxifen therapy. During dose escalation with the aromatase inhibitor exemestane, the methylene line width seemed to decrease (31.9 vs 38.8 Hz, at 12 weeks and baseline, respectively). Significant decreases in total (13%) and HDL (32%) cholesterol, apo A1 (25%), and total triglyceride (16%) levels were found during the same interval. The apo A1/apo B ratio decreased by 25%. For patients on dexamethasone, the proton aliphatic line widths increased one day after the initiation of therapy. The changes in line shape observed during dexamethasone therapy indicated lower levels of triglyceride-rich relative to triglyceride-poor lipoproteins, consistent with results from the lipid analyses. In conclusion, nuclear magnetic resonance spectroscopy might have potential to detect effects on plasma induced by endocrine therapy. The lipid analyses in these patients were in support of the changes in lipid profile as evaluated by nuclear magnetic resonance spectroscopy.