To test the atherosclerosis hypothesis in primary antiphospholipid syndrome (PAPS) we measured intima media thickness (IMT) of carotid arteries and other cardiovascular risk factors in 44 patients with PAPS (mean age 35 +/- 12 years), in 25 patients with inherited thrombophilia (mean age 40 +/- 10 years), and in 34 normal controls (mean age 38 +/- 11 years). The frequency of smoking, hypertension, and dyslipidemia was similar across groups. IMT was almost similar across groups at age groups below 40 years but IMT was greater in PAPS than controls at the common carotid (P = 0.01), at the bifurcation (P = 0.003), and at the internal carotid (P = 0.005) in the age group over 40 years. Atherosclerosis is a possibility in PAPS patients in their fourth decade of life or older.