Event precipitation samples were collected in Chicago, Illinois, and on Beaver Island, Michigan and analyzed for polychlorinated biphenyls (PCBs). The weighted mean concentration of PCBs in 31 samples of rain was 119 ng/L (119 parts in 1012). This would indicate a deposition of 5000kg/yr of PCBs to Lake Michigan from wet precipitation. The concentrations of PCBs in rainfall were found to be as high on Beaver Island as in Chicago. This is interpreted as indicating that the sources of PCBs to the atmosphere are diffuse and/or that residence times in the atmosphere are long. Precipitation is now the major source of these materials to the lake and future problems in the lake with PCBs will be determined mainly by the magnitude of the atmospheric inputs of PCBs to the lake.