The high value of sugar maple logs and lumber depends on the wood being light-colored and clear of defects. Predicting the size of dark hearts in trees before they are harvested is very important to foresters, forest landowners, and sawmills. We investigated many possible predictors of the heart size of sugar maple in 10 sites in New York State. Heart size ratios by site ranged from 12 to 42%, averaging 23%. At the site level, trees with large hearts were more common on more acid soils (P = 0.04). Flaky bark, poor crown ratios, and lower grade stems were correlated with large hearts across the sample of 265 trees. Visible tree injury, competition, and tree diameter were not consistently related to heart size ratios of trees. Steep slopes were associated with large hearts. Other physiographic factors (slope and landform) differed in their effect by site, possibly due to local histories of storm damage. In conclusion, predicting dark heart in sugar maple is likely to remain challenging.