OBJECTIVE AND CONCLUSIONS: This article gives an overview of observational and experimental epidemiological studies relating beta-carotene to risk of cancer and cardiovascular disease. Observational epidemiological studies have consistently shown that a diet rich in beta-carotene-rich fruits and vegetables or high blood levels of beta-carotene are associated with a reduced risk of cancer at a number of common sites, such as lung and stomach. For other cancer sites, such as prostate and breast, the observational evidence is not very consistent or absent altogether. For cardiovascular disease, observational studies are less numerous but do point to a protective effect of high beta-carotene intake. The associations from observational epidemiology may indeed be ascribed to beta-carotene, since a number of plausible preventive mechanisms have been demonstrated for cancer as well as cardiovascular disease. However, observational epidemiology cannot resolve the question whether other constituents from fruits and vegetables or other factors may explain the findings from the case-control and cohort studies. The results of intervention studies undertaken so far are disappointing and do not indicate a preventive potential for beta-carotene. Further intervention trials with longer follow-up may be needed to elucidate whether beta-carotene is protective against certain forms of cancer and against cardiovascular disease.