WHO and UNAIDS advocated healthcare providers to consider provider-initiated HIV counseling and testing for clients attending healthcare facilities. However, there is a lack of evidence, concerning the effectiveness of such interventions in Thailand and other settings with large outbreaks of the HIV epidemic, needed to support policy decisions. A clusterrandomization trial with pre-post test design was conducted to assess the effectiveness of provider-initiated voluntary HIV counseling and testing compared with the current practice in which HIV testing is provided upon the client’s request.