Sitthi-amorn C (Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand), Chandraprasert S, Bunnag S C and Plengvidhya C S. The prevalence and risk factors of hypertension in Klong Toey Slum and Klong Toey government apartment house. International Journal of Epidemiology 1989, 18: 89–94. We have investigated the prevalence and factors associated with hypertension in 976 residents of Klong Toey Slum and 909 residents of government apartment houses, aged 30 and above, selected by probability sampling after systematic household surveys with an average response rate of over 80%. Hypertensives were those who had, on at least three measurements, average diastolic blood pressure equal to or above 95 mmHg and/or systolic blood pressure equal to or above 160 mmHg or had blood pressure below 160/95 mmHg but were currently on antihypertensive medication. The prevalence of total hypertensives were found to be 17.3% and 14.0% for residents of slum and government apartment houses respectively. Men and women had more or less equal mean blood pressure and similar prevalence of hypertension. The mean systolic blood pressure increased with age while the mean diastolic blood pressure, after an initial rise with age in lower age groups, tended to level off from the age group 55–64 years upwards. Only one quarter to one third were aware of their illness and less than 15% were receiving treatment. Significant risk factors include age, duration of smoking, duration of alcohol intake, high body mass index, high Cholesterol, high Triglyceride, high Low Density Lipoprotein Cholesterol (LDLC), low High Density Lipoprotein Cholesterol (HDLC), high Total Cholesterol (TC) to High Density Lipoprotein ratio (TC/HDLC), high LDLC to HDLC ratio and diabetes mellitus. The data suggested that hypertension was an important public health problem in low socioeconomic groups in Bangkok. Some of the risk factors were related to an unhealthy lifestyle which should receive due consideration in planning for appropriate control.