Increased anthropogenic mercury (Hg) deposition since pre-industrial times, and subsequent transformation of inorganic Hg to methylmercury (MeHg) in aquatic environments, has created areas in North America where Hg poses a relatively high risk to wildlife, especially long-lived, piscivorous species. From 1995 to 2001, we opportunistically collected 577 eggs abandoned by Common Loons from eight states. Egg-Hg concentrations ranged from 0.07 to 4.42 microg/g (ww) or 0.10 to 19.40 microg/g (dw). Mercury was higher in eastern than in western North America. Female blood-Hg concentrations strongly correlated with those of eggs from the same territory even though the mean intraclutch Hg difference was 25%. In New England, egg volume declined significantly as egg-Hg concentrations increased. Fertility was not related to egg-Hg concentrations. Based on existing literature and this study's findings, egg-Hg risk levels were established and applied to our US data set and an existing Canadian data set. Regionally, we found the greatest risk levels in northeastern North America. With few exceptions, loon eggs are suitable indicators of methylmercury availability on lakes with territorial pairs.