Social support is an important factor for exercise among cancer patients, but too much control might elicit reactance and lead to detrimental effects. In this pilot study, 56 dyads (cancer patient + relative) filled out a questionnaire assessing social support, social control, and reactance. After 4 weeks (T2), patients’ exercise was assessed with a 7-day recall. About half of the patients did not engage in any self-reported exercise behavior. Relative-reported support was the only variable associated with exercise behavior at T2. Perceived control ( r  = .4) but not perceived support was significantly correlated with reactance. Male patients reported more support, but were also more prone to reactance.