The insulin resistance syndrome is characterized by a constellation of risk factors including obesity, central body fat distribution, hypertension, glucose intolerance, elevated plasma insulin levels, increased triglyceride and decreased high-density lipoprotein (HDL) cholesterol. Similarly, low-density lipoprotein (LDL) subclass phenotype B, characterized by a predominance of small, dense LDL particles, is associated with increased triglyceride and lower HDL cholesterol levels. Both the insulin resistance syndrome and phenotype B have also been related to increased risk of coronary heart disease. Using a sample of nearly 700 women who participated in the Kaiser Permanente Women Twins Study, we investigated the associations of LDL subclass phenotype B with the risk factors that characterize the insulin resistance syndrome. The results demonstrate that LDL subclass phenotype B was strongly associated with both age and diabetes status. Among nondiabetic women, phenotype B was more prevalent in those with higher body mass index and in those with higher waist-hip ratio. In addition to the expected associations with triglyceride and HDL cholesterol, mean values of both systolic and diastolic blood pressure were higher in women with phenotype B. Finally, fasting and post-load insulin levels and post-load glucose levels were higher in those with phenotype B. In general, these relationships persisted in multivariate statistical analyses. Therefore, a predominance of small, dense LDL particles appears to be an integral feature of the risk factors that characterize the insulin resistance syndrome.