Few studies have examined the relationship between degree of lesion in various locations and improvement during treatment in stroke patients with chronic aphasia. The main purpose of this study was to determine whether the degree of lesion in specific brain regions was related to magnitude of improvement over the course of object and action naming treatments. Participants and methods: Fifteen left hemisphere stroke patients with aphasia participated in treatments for object and/or action naming. Two raters assessed extent of lesion in 18 left hemisphere cortical and subcortical regions of interest (ROIs) on CT or MRI scans. Correlations were calculated between composite basal ganglia, anterior cortical, and posterior cortical lesion ratings, on the one hand, and both pretreatment scores and treatment change for both object and action naming, on the other hand. Results: Unexpectedly, greater anterior cortical lesion extent was highly correlated with "better" object and action naming scores prior to treatment and with greater improvement during treatment when partial correlations controlled for total basal ganglia lesion extent (r ranging from 0.730 to 0.858). Greater total basal ganglia lesion extent was highly correlated with "worse" object and action naming scores prior to treatment and with less improvement during treatment when partial correlations controlled for total anterior lesion extent (r ranging from -0.623 to -0.785). Correlations between degree of posterior cortical lesion and naming indices generally were not significant. No consistent differences were found between the correlations of ROI lesion ratings with object naming versus action naming scores. Conclusion: Large anterior cortical lesions and intactness of the basal ganglia may both contribute to more efficient reorganization of language functions. (Contains 6 tables.)