Nutritional status and household food security were longitudinally monitored in three besieged cities of Bosnia-Hercegovina (Sarajevo, Zenica and Tuzla) during the winter and spring of 1993 to 1994. The objectives were to provide early warning of a deterioration in the food and nutrition situation and identify particularly vulnerable groups so that action could be taken to prevent potential undernutrition and target resources to the most needy. Before the cease-fire, which came into effect at the end of February 1994, trends in various indicators (weight loss, decline of household food stocks, rising food prices, reduction in food aid distribution, sale of possessions) suggested that the situation was deteriorating. Access to food improved as a result of the cease-fire, however, which was reflected in improvements in indicators of food security and weight gain. The impact of, and response to, the food emergency differed between individuals, households and locations; the elderly were found to be more nutritionally vulnerable than children or adults, households with the least access to resources were the most food insecure, while the city of Zenica appeared to be particularly hard hit. The findings show the effects of an uncertain and reduced food supply on a previously well-fed healthy population in an industrialised country. The value and constraints of an Early Warning System set up to monitor trends in an emergency in a European context are discussed.