The Honduras AIN-C programme is a preventive health and nutrition programme of the Honduras Ministry of Health (MOH) that relies on volunteers to help mothers monitor and maintain the adequate growth of young children. A quasi-experimental, design-based evaluation found that the programme achieved near-universal coverage and was effective in improving mothers' child-rearing knowledge, attitudes and practices, including feeding and appropriate care-giving and care-seeking practices for children with diarrhoea and acute respiratory illness. The programme is widely regarded as a model. This study was undertaken to provide the first comprehensive estimates of the cost of the AIN-C programme, with the goal of providing a programme and financial planning tool for Honduras. An additional comparison of study findings was also undertaken to determine the cost of the AIN-C programme's community-based services relative to a similar facility-based service. Expressed in mid-2005 US dollars, the study found that after the programme is phased-in: (1) the annual, recurrent cost per child under 2 years participating in the programme is $6.43; (2) the annual, incremental budget requirements per child under 2 years participating in the programme are $3.90; (3) the cost of an AIN-C monthly growth monitoring and counselling session per child is 11% of the cost of a traditional MOH, facility-based growth and development consultation per child; and (4) the effect of mothers substituting AIN-C monitor care for MOH facility-based care 'saves' 203 000 outpatient visits a year, with a potential cost saving of $1.66 million, the equivalent of 60% of the recurrent cost of the programme and roughly equal to the annual incremental budget requirements of the programme. Sensitivity analysis of the cost estimates is performed to provide insight, for countries considering introducing a similar programme, into how modifications of key characteristics of the programme affect its costs.