OBJECTIVE: To preliminarily investigate the prevalence of HIV co-infection in patients with malaria in Mae Sot District, Tak Province of Thailand. METHODS: The study was a retrospective study on blood samples collected from a total of 256 patients with malaria (all species and severity) who attended Mae Tao clinic for migrant workers, Tak Province during 2005-2007 (148 samples) and 2010-2012 (108 samples). Malaria diagnosis was performed based on microscopic examination of patients' blood smears. Chemiluminescent microparticle immunoassay and gel particle passive agglutination were employed for the detection of HIV antigen in patients' plasma. RESULTS: Plasmodium falciparum (P. falciparum) and Plasmodium vivax (P. vivax) are the two predominant malaria species with the ratio of about 1: 1 to 1.5:1. Most of the P. falciparum cases were presented with acute uncomplicated signs and symptoms with highest parasitemia of 1 045 000 asexual parasites/μL bloods. The prevalence of malaria and HIV co-infection during 2005-2007 was 1.35% (2/148 cases, 1 each for P. falciparum and P. vivax co-infection), but was increased to 2.78% (3/108 cases, 2 and 1 for P. falciparum and P. vivax co-infection, respectively) during 2010-2012. CONCLUSIONS: The increasing trend of prevalence of malaria and HIV co-infection in Mae Sot, Tak province was of a great concern on either pharmacodynamics or pharmacokinetics aspect. The study in a larger numbers of malaria patients in different endemic areas throughout the country with different time periods is underway.