This paper reports a screening survey using a mobile unit to determine the prevalence of chronic disorders among persons aged 40-59 years in rural Thailand. A total of 4,812 persons from all the 54 rural villages in Mae Sot District, Tak Province, northern Thailand, were interviewed and examined in 1995. A higher proportion (59.3%) of men were current tobacco smokers than among women (40.4%). A higher proportion (69.6%) of men were current alcohol drinkers compared with women (38.6%). Cutting/piercing was the most frequently reported injury, followed by falls. About half (47.9%) of the persons surveyed had a body mass index (BMI) between 20 and 24.9 and only 2.6 per cent had a BMI of 30 or over. The overall prevalence rates of hypertension and diabetes in the persons surveyed were 13.3 per cent and 2.4 per cent respectively. Of the persons screened, 28.8 per cent had borderline-high blood cholesterol (200-239 mg/dl) and 12.9 per cent had high blood cholesterol (> or = 240 mg/dl). About 61 per cent of hypertensive persons, 92 per cent of diabetic persons, and nearly all of those with dyslipidaemia were first detected during this screening programme. Five women with breast cancer and 22 with benign breast disorders were also identified during the survey. A screening programme using a mobile unit may be useful in identifying treatable disorders in rural areas, where existing screening services cannot effectively cover the population at risk.