The prevalence and clinical associations of beta-migrating very-low-density lipoprotein (beta-very-low-density lipoprotein), an atherogenic lipoprotein associated with type III hyperlipidemia, were compared in men and women using and not using estrogen-containing hormones. Beta-very-low-density lipoprotein was more common in men, 0.54 percent or one in 185, than in women not taking hormones, 0.30 percent or one in 333, and was least common in women taking hormones, 0.04 percent or one in 2,500. Beta-very-low-density lipoprotein-positive men and women were heavier and had higher triglyceride, cholesterol, and very-low-density lipoprotein cholesterol levels compared with beta-very-low-density lipoprotein-negative subjects, but 30 percent of the men and 21 percent of the nonuser women did not have elevated triglyceride or cholesterol levels. The very-low-density lipoprotein cholesterol/total triglyceride ratio exceeded 0.30 in 55 percent and 54 percent of beta-very-low-density lipoprotein-positive men and women compared with 3.3 percent and 3.9 percent of beta-very-low-density lipoprotein-negative men and women. Ninety-three percent of the beta-very-low-density lipoprotein-positive subjects had some lipoprotein lipid abnormality and 60 percent had an abnormal glucose, liver, or renal function test result or high alcohol or dietary cholesterol intake. In conclusion, beta-migrating very-low-density lipoprotein is more common in men than in women, consistent with a protective effect of estrogen. About half of the time, beta-very-low-density lipoprotein is associated with an elevated very-low-density lipoprotein cholesterol/total triglyceride ratio, indicating that beta-very-low-density lipoprotein can be present independent of other markers of type III hyperlipidemia. Nonetheless, beta-very-low-density lipoprotein signifies abnormal lipoprotein metabolism of some kind in almost every instance, is frequently associated with treatable medical conditions, and may on pathophysiologic grounds be considered important in its own right in predicting atherosclerosis risk. Beta-very-low-density lipoprotein-associated conditions should attract medical attention.