This paper applies cost-efficiency analysis to an intervention that promotes behavior change for rural sanitation in Tanzania. The campaign targets a number of potential beneficiaries, out of which some are effectively encouraged to adopt the new practices (beneficiaries). As a result, the cost-efficiency of the program depends on the extent of take-up of improved sanitation by the target population, unknown in advance. To correctly account for the costs of households gaining access under this demand-driven approach, both costs (investments) and increased access to sanitation are considered outcomes and are estimated from samples of beneficiary and control populations, using a randomized-controlled trial design. Results show that sanitation promotion did not lead to higher investment relative to the control group and that the cost-per-person effectively gaining access to sanitation is substantially higher than the cost-per-person targeted or at-reach of the campaign. Using these estimates, the authors found that universal coverage can be obtained for the equivalent of 4 per cent of Tanzania's national GDP (2013). They also used parameters estimated from the study to simulate cost-per-person of the program when take-up increases (efficiency gains).