Potential fire behavior was compared under dry, windy weather conditions in 12 ponderosa pine stands treated with alternative thinning prescriptions in the wildland/urban interface of Flagstaff, Arizona. Prior to thinning, stands averaged 474 trees/ acre, 158 ft^2/acre basal area, crown bulk density 0.0045 lb/ft^3, and crown base height 19.2 ft. Three thinning treatments differing in residual tree density were applied to each of three stands (total of nine treated, three control). Treatments were based on historic forest structure prior to Euro-American settlement and disruption of the frequent fire regime (circa 1876). Thinning reduced stand densities 77-88 percent, basal areas 35-66 percent, crown bulk densities 24-48 percent, and raised crown base height an average of 11 ft. Before thinning, simulated fire behavior under the 97th percentile of June fire weather conditions was predicted to be intense but controllable (5.4 ft flame lengths). However, active or passive crownfires were simulated using crown base heights in the lowest quintile (20 percent) or winds gusting to 30 mph, representing the fuel ladders and wind gusts that are important for initiating crown burning. Under the identical conditions after thinning, all three treatments resisted crown burning. The degree of resistance was related to thinning intensity. It is crucial to remove thinning slash fuels through prescribed burning or other means. If not removed, slash fuels can cause crownfire behavior in the thinned stands under severe wildfire conditions. Finally, the crownfire resistance achieved through thinning will deteriorate over time unless maintenance burning and/or thinning is continued.