OBJECTIVE--To assess the effect of workplace-oriented dietary intervention on serum lipids. DESIGN--A cohort study. Screening for blood lipids and subsequent dietary intervention performed at a ferro-alloy factory where all the workers have at least one meal per working day served by the factory kitchen. SETTING--Employees at Grundartangi ferro-alloy factory, Iceland. SUBJECTS--From 1 June 1989 to 1 June 1991, 155 workers participated in the screening and prevention programme. INTERVENTION--Changing the food served by the kitchen to 200 workers by lowering the total number of calories and increasing the content of fibre and unsaturated fats in the diet. RESULTS--The mean serum cholesterol was lowered from 6.64 to 6.09 mmol/l or by -8.28% for the whole group (p