Goals The aim of this study was to examine the effect of a cancer diagnosis on the health behaviors of cancer survivors and their family and friends, and to determine whether a cancer diagnosis could be a teachable moment for intervention. Materials and methods This was a cross-sectional study of the health behaviors of individuals taking part in a cancer fundraising event. The questionnaire was completed by 657 participants. Main results Participants were 81.4% women, had a mean age of 46 years, and comprised of 17.2% cancer survivors. For cancer survivors, 31.3% reported an increase in physical activity, 50% of smokers quit, and 59 to 72% reported dietary improvements within 1 month of diagnosis. Significant differences in behavior change were found by age, but not by gender or education. For individuals without cancer, 24.3% reported improved physical activity and the majority reported some dietary changes. A greater proportion of family and friends who perceived they were at greater risk of developing cancer increased physical activity and sun-smart behavior but did not improve dietary habits. Conclusions The results indicate that the cancer survivors made significantly more positive health behavior changes compared to the non-cancer group. For this sample, a personal diagnosis of cancer, or a diagnosis in a family member or friend, may have acted as a 'cue to action' to improve lifestyle health behaviors. This field of research is still at an early stage, and further studies are needed to confirm if this situation could be useful as a 'teachable moment' for intervention purposes.