To advance our understanding of an association between exposure to power frequency magnetic fields (MFs) and the risk of childhood leukemia, we should conduct a study that is convincingly free of selection and response bias, with highly accurate exposure assessment and a large number of highly exposed individuals. Previous measurements revealed that MF in apartments located above internal transformer stations (ITSs) are higher than in other apartments in the same building. An international epidemiologic study of childhood leukemia, TransExpo, was designed to take advantage of this scenario. This article presents the results of an exposure assessment study performed in apartment buildings with ITS in Israel. Measurements were performed in 41 apartments within 10 buildings. Average MF at the height of 0.5 m was 0.40 μT in apartments above the ITS and 0.06-0.12 μT in all other apartments. These results confirm that classification of MF exposure based on apartment location is feasible with remarkable specificity (0.98 and 0.96 for cutoff points of 0.2 and 0.4 μT, respectively) and sensitivity (1.00 for both cutoff points). Because the location of an apartment relative to the ITS can be easily determined, an exposure assessment can reliably be performed without obtaining access to residences.