We examined whether N fertilization of Douglas-fir (Psuedotsuga menziesii [Mirb.]) plantations in western Washington 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% more C (average 24 Mg/ha) than the control plots in the unthined and 22.5% more C (average 38 Mg/ha) than the control plots for the thinned stands. This study suggests that N fertilization of commercial forests in western Washington could substantially increase C stored in these forest ecosystems, though the absolute effect of widespread fertilization on stands of varying properties was not evaluated in this study.