Although the United Mine Workers of America (UMWA) has always advocated strongly for miners’ safety, prior empirical literature contains no evidence that unionization reduced mine injuries or fatalities during the 1970s and ‘80s. This study uses a more comprehensive dataset and updated methodology to examine the relationship between unionization and underground, bituminous coal mine safety from 1993 to 2010. I find that unionization predicts a substantial and significant decline in traumatic injuries and fatalities, the two measures that I argue are the least prone to reporting bias. These disparities are especially pronounced among larger mines. My best estimates imply that overall, unionization is associated with a 13-30% drop in traumatic injuries and a 28-83% drop in fatalities. Yet unionization also predicts higher total and non-traumatic injuries, suggesting that injury reporting practices differ between union and nonunion mines.