The levels of pro- and antiatherogenic lipoproteins are the most important risk factors for vascular disease, and there is now compelling evidence that the apolipoprotein (apo) B/apoA-I ratio is a better index of the likelihood of vascular events than any of the corresponding cholesterol indices: the total cholesterol/high-density lipoprotein cholesterol (HDL-C) ratio, non-HDL-C/HDL-C ratio, or low-density lipoprotein cholesterol (LDL-C)/HDL-C ratio. But are there any restrictions on the application of the apoB/apoA-I ratio to clinical practice? This article suggests that the answer is yes. Based on the available biologic and epidemiologic data, the relation between risk and apoB is continuous, whereas at the extremes of HDL concentration in plasma the relation to risk is not certain. Moreover, LDL plays a causal role in atherogenesis whereas HDL plays a contingent role. Appreciating these distinctions should allow appropriate use of the apoB/apoA-I ratio as a simple, single, summary index of the lipoprotein-related risk of vascular disease.