Objectives of this study are to verify the quality of data from the Survey of Commercial Sex Institutions and Number of Commercial Sex Workers carried out by Venereal Diseases Division, Ministry of Public Health, and to examine factors which have impact on the data set based on data in 1995 and 1996. Samples included 1) officers collecting data of commercial sex institutions and commercial sex workers in Chiangrai province between 1995 and 1996 for their work process, procedures and various factors which may have affected the quality of data shown in the report on the number of commercial sex institutions and commercial sex workers in Chiangrai province 1995. 2) local individuals having indirect role in data collection of commercial sex institutions and commercial sex workers but were familiar with commercial sex business in such locations. Attitudes of the latter group are reflected in their perceived reliability and quality of data in the report of Venereal Diseases Division, Ministry of Public Health. For this study, 4 districts of Chiangrai province were selected. Qualitative method and content analysis were utilized. The results of this study show that groups of local individuals who are not directly involved with data collection of commercial sex institutions and commercial sex workers have doubt in the Chiangrai province data reported by Venereal Diseases Division, Ministry of Public Health 1995. The study found factors which have little effect on the quality of data among officers who carried out data collection.Such factors include work motivation, attitudes, work satisfaction, manpower as related to work load, data collection skills, personnel selection and work experience. Moreover, factors of transferal of data, technical supervision and training before data collection have no significant effect on the quality of reported data. On the other hand, factors found to have adverse effect on quality of data exist in various stages of actual report development which are area coverage, recording accuracy, data management and analysis.