A 36-year-old woman was treated with tamoxifen for lung metastasis of breast cancer and had marked hyperlipoproteinemia with giant fatty liver, high plasma triglyceride levels (3673 mg/dl), and increased levels of very low density lipoprotein (VLDL) and intermediate density lipoprotein (UDL). A low level of activity of both plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) was also noted. Our observations support the concept that, in some patients, the weak estrogen-like activity of tamoxifen is amplified and, in severe lipemia, reduction of the activities of LPL and HTGL might impede the conversion of VLDL to LDL, thus causing the amplification of the effect.