Background:Methods(gradient gel electrophoresis or density-gradient ultracentrifugation) for detecting small dense low dense lipoprotein(sd-LDL) are too time-consuming or expensive. We report a novel and simple formula for LDL /LDL-apoB as an index of sd-LDL using plasma levels of total cholesterol (TC), high density lipoprotein (HDL), LDL and apo B. Methods:We measured LDL peak particle diameter by gradient gel electrophoresis in 78 patients (52.5짹13.4 years, male 37.2%) without cardiovascular disease (CVD). Pattern A was defined as a diameter of >25.5 nm, and pattern B (dominant sd-LDL) was defined as a diameter of ���25.5 nm. Applying a Friedwald`s formula, we developed a formulae for LDL-apo B {(total apoBxLDL)/(TC–HDL)}, and LDL/LDL-apo B {(TC-HDL)/total apoB}. We compared LDL/LDL-apo B value in pattern A group with that in pattern B group. We tried to find cutoff value of LDL/LDL-apo B as an index of pattern B using ROC curve. Results:There was a significant difference in LDL/LDL-apo B value between pattern A and pattern B (respectively 1.71짹0.27, 1.49짹0.14, p=0.034). Lower LDL/LDL-apo B ratio reflected preponderance of sd-LDL. Cutoff value of LDL/LDL-apo B predicting sd-LDL in ROC curve was less than 1.6(AUC=0.832,sensitivity=87.5%,specificity=60%,Figure). Conclusions:LDL/LDL-apo B ratio reflects the size of LDL and preponderance of sd-LDL is postulated when the ratio falls below 1.65. We suggest that our new formula for LDL/LDL-apoB is a simple and inexpensive screening test for predicting sd-LDL in patients without CVD.