The severity of the 2000 fire season has increased public awareness of a widespread fuels problem in western U.S. forests. Federal land management agencies have responded with plans to greatly expand programs to mitigate hazardous fuel conditions. However, scant information exists on the efficacy of fuel treatments for mitigating wildfire severity. There is even less information regarding the influence of fuel treatments on second-order fire effects, such as invasive plants. A unique opportunity to address this research void has been provided by the 2000 Cerro Grande Fire near Los Alamos, New Mexico.The Cerro Grande Fire burned over multiple dispersed stands that had previously been treated with a variety of methods, which included thinning with and without slash removal. Immediate post-fire sampling focused on fire severity comparisons in adjacent treated and untreated stands. Ratings of crown damage and ground char were lower in treated stands than in adjacent untreated stands (paired t-test, P < 0.05). We attribute these differences primarily to lower tree density in treated stands (P < 0.05). Reductions in fire severity were somewhat greater in thinned stands where slash was left than in thinned stands where slash was removed, though differences between these treatment types were not significant. This mixed result may be due to chance but could imply that, under extreme fire conditions, stand density may be a more important determinant of fire behavior and effects than surface fuel load.We greatly expanded our sampling effort in the summer of 2001 with a stratified random sampling design and multiscale plots to assess relationships among fuel treatments, stand conditions, fire severity, and invasive plants. Strata included aspect, elevation, cover type, fuel treatment type, and fire severity. Preliminary results indicate that percent relative invasive cover is higher in ponderosa pine (Pinus ponderosa) stands compared with other vegetation types sampled, though lower in fuel-treated areas. Relative invasive cover was also highest in severely burned areas and lowest in unburned stands. Further, the invasive plant species threat is lowest in stands that were both thinned and burned. Continuing analyses should improve our understanding of the potential ecological consequences of expanded fuel treatment activities. © 2004, Tall Timbers Research, Inc.