PURPOSE: Homosexual and bisexual men are at an increased risk for human papillomavirus-induced squamous intraepithelial lesions and cancer of the anus. Our objective was to estimate the cost-effectiveness of screening for anal squamous intraepithelial lesions in these high-risk patients. SUBJECTS AND METHODS: A Markov model was developed to evaluate alternative screening strategies using anal cytology in a hypothetical cohort of homosexual and bisexual men. Data were obtained from prospective cohort studies, national databases, Medicare reimbursement rates, and the published literature. Model outcomes included life expectancy, quality-adjusted life expectancy, total lifetime costs, and incremental cost-effectiveness ratios. RESULTS: The undiscounted life expectancy gain associated with anal cytology screening every 3 years was 5.5 months. Compared with no screening, screening every 3 years increased the discounted quality-adjusted life expectancy by 1.8 months and cost $7,000 per quality-adjusted life year (QALY) gained. Screening every 2 years cost $15,100 per QALY gained compared with screening every 3 years. Annual screening provided incremental benefits of less than 0.5 quality- adjusted months and had an incremental cost of $34,800 per QALY gained. Screening every 6 months provided little additional benefit (i.e, 5 days) over that of annual screening and had an incremental cost of $143,500 per QALY gained. CONCLUSION: In homosexual and bisexual men, screening every 2 or 3 years for anal squamous intraepithelial lesions with anal cytology would provide life-expectancy benefits comparable with other accepted preventive health measures, and would be cost-effective. (C) 2000 by Excerpta Medica, Inc.