Patients with cystic fibrosis have fat malabsorption, providing an experimental model for evaluation of the hypothesis that a low-fat intake may prevent atherosclerosis. We studied the frequency and extent of aortic precursor lesions (fatty streaks, early fibromusculoelastic lesions, late fibromusculoelastic lesions) found at autopsy in this disease as well as in other patients with debilitating disorders but with no apparent impairment of fat absorption. Fatty streaks were less common in the cystic fibrosis group, as were the late fibromusculoelastic lesions. There was no significant difference in the frequency, length, or thickness of the early fibromusculoelastic lesions. The findings suggest that fat may be responsible for progression but not initiation of the fibromusculoelastic precursor lesions, and support the concept that early restriction of dietary fat may prevent, delay, or otherwise modify atherosclerosis in the adult.