The Italian-American Clinical Trial of Nutritional Supplements and Age-Related Cataract (CTNS) is a 13-year study designed primarily to evaluate the safety and efficacy of a vitamin-mineral supplement containing recommended daily allowance (RDA) dosages in preventing age-related cataract or delaying its progression. As secondary objectives the study will collect data on incidence and progression rates as well as risk factors for the disease. The clinical trial was initiated largely because of epidemiological studies suggesting that various nutrients, particularly those with antioxidant capabilities, might retard cataract development. The possibility of a beneficial effect on cataract and other age-related diseases has contributed to the widespread use of dietary supplements in the United States among the elderly population, even in the absence of definitive evidence about the safety and effectiveness of such use. The low rate of dietary supplement use in the Italian population provided an opportunity for a trial in which the efficacy of RDA dose supplementation could be tested against no supplementation at all. Relevant design issues for the study include defining cataract and cataract progression, estimating event rates, evaluating reproducibility of the lens grading system for different types of opacity, and identifying the criteria for assessing efficacy and safety. This paper describes the CTNS design and rationale and the approach used to address the issues described above.