Objectives: Cognitive training has been shown to improve both cognitive and everyday abilities in older adults; however, little is known concerning the amount of training needed or the characteristics of those who benefit. These analyses examined the longitudinal impact of dosage (number of training sessions) on the improvement and maintenance of cognitive and everyday function. Methods: ACTIVE is a longitudinal, randomized, single-blind clinical trial evaluating cognitive interventions in older adults (aged 65-94) from six states in the United States. Results: Latent growth curve models indicated that initial training effects were maintained over 5 years and amplified by booster sessions. A single booster session counteracted 4.92 months of age-related processing speed decline. Discussion: Cognitive performance improved by 2.5 standard deviations for participants who attended all 10 initial sessions and all 8 booster sessions compared to randomized participants who attended none. Implications for the broader application of cognitive training interventions are discussed. © 2013, SAGE Publications. All rights reserved.