Old-growth forests are currently identified as core components of regional conservation and forest-reserve planning efforts by agencies and organizations across the northeastern United States. Despite the importance of these ecosystems from an ecological and conservation standpoint, major questions remain concerning their actual extent, location, and configuration in many states. Here we report a substantially revised estimate for individual tracts and the total area of old-growth forests in Massachusetts based on analysis of historical documents and extensive field research and mapping. We estimate that the total area of old-growth in the state is 453 ha, in 33 stands that range from 1.2 to 80.9 ha in size. Over 80% of these forests occur in the Berkshire Hills and Taconic Mountains in the extreme western part of the state. These forests are structurally unique and contain some of the oldest documented Tsuga canadensis (hemlock) and Picea rubens (red spruce) in New England, as well as the second-oldest documented Betula lenta (black birch) in the country. Due to their relatively small size and isolated character, these areas are susceptible to human and natural disturbance and require protection, including substantial buffer areas. Old-growth stands will enhance the value and function of designated forest reserves and will gradually become surrounded by forests of increasingly similar structure and ecosystem characteristics.