Between February 1990 and December 1995, professionals diagnosed six cases of childhood leukemia among residents of the small rural community of Elbmarsch in northern Germany. Five of these cases were diagnosed in only a 16-mo period between February 1990 and May 1991. All cases lived in close proximity (i.e., 500-4,500 m) to Germany's largest capacity nuclear boiling-water reactor. We calculated standardized incidence ratios and exact 95% confidence intervals for a 5-km-radius circular area around the plant. The standardized incidence ratio for the time period 1990-1995 was 460 (95% confidence interval: 210, 1,030). The analysis was restricted further to the years 1990 and 1991, and the standardized incidence ratio increased to 1,180 (95% confidence interval: 490, 2,830). Presently, this cluster of childhood leukemia cases cannot be explained in terms of established and putative risk factors--including radiation from medical sources--for childhood leukemia.