In Europe household appliances are a major source of indoor 50 Hz magnetic field exposure. A number of epidemiological studies have reported associations between leukemia risk and personal use of household appliances. In the "Norddeutsche Leukämie und Lymphomstudie" (NLL), which was conducted in Northern Germany (Schleswig-Holstein and Lower Saxony) between 1997 and 2001, lifetime use of a preselected array of electric appliances (microwave ovens, hair-dryers, motorized electrical alarm clocks (i.e. that use a motor to move their hands or digits), electric blankets and pillows, heated waterbeds, computers with conventional screens, TVs, and electric sewing machines) was recorded in a standardized, personal, computerized interview. Exposure was assessed on three different levels of precision: ever use, cumulative appliance-years, and average time of daily use. Additional questions referred to exposure modifying factors, including distance from screen while watching TV, position of an alarm clock at the bed etc.). Exposure to ELF-EMF from household appliances was quantified as ever vs. never use, gross and net appliance-years of lifetime use and cumulative microT-hours. Flux densities were based on measurements of appliances from the published literature. These were used as weighting factors to account for the different device-specific contributions to overall ELF-EMF exposure. Resulting distributions (as quartiles) for exposure scores revealed systematic differences for different levels of precision. Our analysis indicates that valid assessment of ELF-EMF exposure from household appliances should be based on the highest possible degree of precision and hence provides a considerable challenge in analytic epidemiology.