The "3 by 5" goal to have 3 million people in low and middle income countries on antiretroviral therapy (ART) by the end of 2005 is ambitious. Estimates of the necessary resources are needed to facilitate resource mobilisation and rapid channelling of funds to where they are required. We estimated the financial costs needed to implement treatment protocols, by use of country-specific estimates for 34 countries that account for 90% of the need for ART in resource-poor settings. We first estimated the number of people needing ART and supporting programmes for each country. We then estimated the cost per patient for each programme by country to derive total costs. We estimate that between US5.1 billion dollars and US5.9 billion dollars will be needed by the end of 2005 to provide ART, support programmes, and cover country-level administrative and logistic costs for 3 by 5.