Stem maintenance respiration was linearly related to live-cell volume for lodgepole pine (Pinuscontortavar.latifoliaEngelm.) from 4 to 36 cm dbh and for Engelmann spruce (PiceaengelmanniiParry) from 0 to 20 cm dbh. Sapwood contained greater than 80% of the total live-cell volume in stems. Bole surface area, commonly used to estimate tree respiration costs, poorly estimated stem maintenance respiration. At 15 °C, maintenance costs for lodgepole pine were 6.6 × 10−5 kg C•(kg C sapwood)−1•d−1. Stem respiration during the growing season, both corrected and uncorrected for maintenance, correlated well with annual stemwood growth. Annual stem maintenance respiration for trees and stands can be estimated using sapwood volume, sapwood temperature, and knowledge of respiratory behavior. Total respiration (construction plus maintenance) estimated using stem growth and a model of maintenance respiration was compared with actual respiration measurements integrated over a 100-d growing season. Estimated respiration agreed with the integrated measurements for Engelmann spruce, but overestimated the integrated measurements by 73% in lodgepole pine. These results suggest that estimates of stem respiration made during the growing season may be affected by transpiration.