Osteoarthritis is a common, long-term, degenerative joint disease often affecting the hips and knees. Aerobic, strength and stretching exercise programmes have been shown to improve function in people with osteoarthritis, but their full benefits are limited by poor adherence. Action and coping plans, a central component of the Health Action Process Approach (HAPA) model of behaviour change, have been shown to improve exercise adherence behaviour in people with long term disorders. Therefore the purpose of this study was to investigate the effect of action and coping plans on exercise adherence behaviour in people with osteoarthritis of the hip and or knee. Twenty seven people with moderate osteoarthritis of the knee and or hip were randomly allocated to the exercise group plus action and coping planning (intervention) or the exercise group only (control). Fifteen of these people completed the programme. Both groups attended a lower limb exercise class three times per week for a period of 12 weeks and completed a home-based walking and stretching programme. The outcome measures were adherence measured throughout the exercise programme and self-efficacy and functional outcomes measured at the beginning and the end of the programme. Adherence was assessed class attendance and adherence to the class- and home-based exercise programmes. Self-efficacy was measured by phase specific self-efficacy (task, maintenance and recovery) and the Arthritis Self-Efficacy Questionnaire. Functional outcomes were actual functional performance (TUG, 6MWT, step test and 10MWT), perceived functional performance (LLTQ-ADL), and pain. The group comparisons were analysed using analysis of variance, and correlations were analysed using Pearson correlation coefficients and regression analyses where appropriate. There were no significant differences between the two groups’ rates of class attendance (p=.811), class-based exercise adherence (p=.522), home-based exercise (p=.209) and walking adherence ( p=.927). There were no significant differences in the self-efficacy scores of the control group over the time of the study. In comparison to the control group, the intervention group’s Arthritis Self-Efficacy Questionnaire function subscale scores were significantly higher post-study (p=.015), but their maintenance self-efficacy scores were significantly lower post-study (p=.025). Significant differences in the actual functional performance measures occurred between the two groups from pre- to post-study, with the intervention group improving significantly on the TUG (p=.005), step test (p<.0005) and the 10MWT (p=.007), but the control group improved significantly only on the 10MWT (p=.029). There was significant difference in the action and coping plans group’s perceived functional performance measures from pre- to post-study (p=.007), but not for the control group (p=.460). Notable significant correlations occurred between; pre-study Arthritis Self-Efficacy Questionnaire other activities subscale and home walking adherence (r= .43); post-study LLTQ-ADL, and the pre-study task self-efficacy and Arthritis Self-Efficacy Questionnaire function and other activities subscales; pre-study Arthritis Self-Efficacy Questionnaire function subscale and TUG; SIRAS and the post-study LLTQ-ADL. Action and coping plans in combination with an exercise programme improved actual and perceived functional performance and self-efficacy to some extent, but did not improve adherence behaviour. A limitation of this study was the small sample size, and a larger study is needed to test the full value of action and coping plans on people with osteoarthritis of the hip and/or knee.