Associations between certain lifestyle characteristics and prostate cancer risk have been reported, and continuation post-diagnosis can adversely affect prognosis. We explored whether men make spontaneous changes to their physical activity and alcohol intake, body mass index (BMI) and smoking status, following a diagnosis of localised prostate cancer. A detailed diet, health and lifestyle questionnaire was completed by 511 participants within the Prostate Testing for Cancer and Treatment (ProtecT) randomised controlled trial, both before and 9 months after a diagnosis of prostate cancer. Of 177 men who were insufficiently active before their diagnosis (median 0 activity units/week; IQR 0-9), 40.7% had increased their activity by a median of 22 U week(-1) (IQR 15-35) 9 months later, and there was weak evidence that men were more active after diagnosis than before (p = 0.07). Men categorised as "working" occupational social class and who were insufficiently active before diagnosis were 2.03 (95%, CI = 1.03-3.99, p = 0.04) times more likely to have increased their physical activity levels compared to men classified as "managerial or professional." Similarly, men who were insufficiently active pre-diagnosis and with T-stage 2 compared with T-stage 1 prostate cancer were 2.47 (95%, CI = 1.29-4.71, p = 0.006) times more likely to be sufficiently active post-diagnosis. Following diagnosis, there was an overall reduction in alcohol intake (p = 0.03) and the proportion of current smokers (p = 0.09), but no overall change in BMI. We conclude that some men spontaneously change certain lifestyle behaviours on receiving a diagnosis of prostate cancer. For many men, however, additional support through lifestyle interventions is probably required to facilitate and maintain these changes.