We examined whether N fertilization of Douglas-fir (Psuedotsuga menziesii [Mirb.]) plantations in western Washington and Oregon State could affect C sequestration in trees. Nine unthinned and six thinned sites, which received 1000 kg N ha-1 over a 16-y period, were compared with adjacent unfertilized control sites. Carbon contained in the live trees was estimated using biomass equations and average carbon concentrations. There was more C estimated to be stored in live trees of the fertilized vs. control plots. On average, fertilized trees contained 10.5 percent more C (average 24 Mg/ha) than the control plots in the unthinned and 22.5 percent more C (average 38 Mg/ha) than the control plots for the live trees in the thinned stands. In a smaller study of three stands, an additional 8.0 Mg C/ha accumulated in the soil and other non-live-tree stand components. This study suggests that N fertilization of commercial forests in western Washington could substantially increase C stored in these forest ecosystems. Considering that there are approximately 20.9 million hectares of forest land in the states of Washington and Oregon, fertilization could result in an additional 0.82 Pg of additional carbon sequestrated during a single rotation of forestland, a period of approximately 40 years, for an average of 0.02 Pg/year. How much of this C would offset fossil fuel use in the long-term is unclear.