OBJECTIVES: The Weight Loss Maintenance Trial (WLM) was a multicenter, randomized trial comparing two weight loss maintenance interventions, a personal contact (PC) program with primarily telephone-based monthly contacts, and an Internet-based program (interactive technology, IT), to a self-directed control group, among overweight or obese individuals at high cardiovascular risk. This study describes implementation costs of both interventions as well as IT development costs. METHODS: Resources were micro-costed in 2006 dollars from the primary perspective of a sponsoring healthcare system considering adopting an extant intervention, rather than developing its own. Costs were discounted at 3 percent annually. Length of trial participation was 30 months (randomization during February-November 2004). IT development costs were assessed over 36 months. Univariate and multivariate, including probabilistic, sensitivity analyses were performed. RESULTS: Total discounted IT development costs over 36 months were $839,949 ($2,414 per IT participant). Discounted 30-month implementation costs for 342 PC participants were $537,242 ($1,571 per participant), and for 348 IT participants, were $214,879 ($617 per participant). Under all plausible scenarios, PC implementation costs exceeded IT implementation costs. CONCLUSIONS: Costs of implementing and operating an Internet-based intervention for weight loss maintenance were substantially less than analogous costs of an intervention using standard phone and in-person contacts and are of a magnitude that would be attractive to many health systems, subject to demonstration of cost-effectiveness.