A health diary was used for measurement of illnesses in the northeast rural area of Thailand during the month of November, 1992. Target population were villagers residing in 12 villages which were randomly selected from 2 districts in Khon Kaen province. Three hundred forty-five households (1690 subjects) were selected for the study from all households in those 12 villages. The sample represented 22.2% of all households. Each respondent was instructed how to record illness which may occur among family members during the observation period. Demonstration of recording was performed as well to ensure uniform reporting. Frequency of ill persons among the 1690 members of the selected 345 households was 299 persons (17.7%) in 333 episodes. The ratio of males to females for the reported illnesses was 1:1.18. The most common illnesses were common cold, fever, and abdominal pain, which occurred in 78, 59, and 47 episodes, respectively, followed by headache, and cough. According to the WHO international classification of diseases, diseases of the respiratory system were prevalent (26.1% of total episodes). Types of illness among age groups 0-1 years and 2-5 years were common cold followed by fever. The others two age groups; 16-45, 46-65 years were ill with abdominal pain as the first rank followed by common cold. Fever and common cold were the most frequent illness in age group 6-15 years and the elderly respectively.