Objectives.The authors conducted a randomized controlled trial to evaluate the sustained effectiveness of a computerized reminder system in promoting mammography during a second year of continuing intervention at three primary care practices of a Health Department and a health maintenance organization in Detroit, Michigan.Methods.Out-of-pocket mammography cost was eliminated for all participants (limited intervention). Computer-generated reminders promoting physician referral for mammography were placed in the medical records of women due for mammography 1 month in advance of their due date (full intervention).Results.Among 1,225 year 2 visitors, mammography rates were 44% for full intervention versus 28% for limited intervention at the health department (adjusted odds ratio [OR] for effect of full intervention 1.84; 95% confidence interval [CI]: 1.40-2.40) and 45% for full versus 46% for limited at the health maintenance organization (adjusted OR 1.06; 95% CI 0.80-1.42). These second year results contrasted with those observed for year 1, during which a significant effect of full intervention was demonstrated for both organizations. After controlling for patient characteristics and site, the effect sizes of full intervention were reduced significantly in the second year compared with the first year (P= 0.05).Conclusions.The effect of computerized mammography reminders can be sustained in a second year of continued intervention, but individual practice sites and organizations vary in their responsiveness to the intervention. Strategies to promote periodic and repetitive procedure use must identify and address time-varying barriers to their effectiveness.