Ten methods of calculating heat units for 30-, 40-, 50- and 60-day postbloom periods were applied to air temperatures observed for four years (1980–1984) in ‘Golden Delicious’ and ‘Delicious’ apple orchards in four production areas of the southeast U.S.A. The heat unit sums were correlated with the number of days from full bloom to objectively determined harvest dates to determine the method which provided the greatest significance probability for the correlation coefficient. No one method emerged superior. The resulting regression equations were then applied to data from the 1985 season not used in the development of the prediction equations. Heat unit accumulation equations predicted harvest within 1.0 day for ‘Delicious’ and 8.25 days for ‘Golden Delicious’.