The impact of treatment in the therapeutic communities at Grendon Prison (England) was assessed in terms of changes in personality, hostility and locus of control, and the effect of length of stay was examined. 94 men were tested on reception and discharge from Grendon, using measures that included the Eysenck Personality Questionnaire and Rotter's Locus of Control (I-E) Scale. Mean scores on discharge were significantly closer to normal on all scales, that is, prisoners showed lower levels of Psychoticism, Neuroticism, intropunitive and extrapunitive hostility, higher levels of Extraversion and more internal locus of control. For Neuroticism, the degree of change was significantly related to length of stay. For all scales except Psychoticism, the proportion showing statistically reliable change was greatest for men who had stayed for at least a year. Residence at Grendon is accompanied by changes in personality, hostility and locus of control. The tendency for some changes to be greatest, or to affect more individuals, after relatively long periods at Grendon (in the region of 1-2 yrs) would be in keeping with a progressive treatment effect and supports the results of reconviction studies and also a time-scale suggested by a model of a "therapeutic career." (PsycINFO Database Record (c) 2019 APA, all rights reserved)