Purpose: This study seeks to assess the differences in disease and socioeconomic characteristics, late effects and long-term quality of life (QoL) after head-and-neck cancer (HNC) among participants and non-participants. Methods: Five hundred sixty recurrence-free survivors treated for HNC at least 6 months previously were invited to participate in a repeated measure, controlled intervention study of computerized Patient Reported Outcome (PRO), which is provided to treating physicians at the point of care. Two hundred sixty-six consented to participate, and 292 declined; of those who declined, 103 filled in the baseline questionnaire. Late effects and QoL were evaluated on the EORTC QLQ C-30 and EORTC QLQ H&N35 and HADS questionnaires, and an empirically derived symptom list was prepared with hospital clinicians. Results: Participants were younger, had a higher educational level, were more likely to cohabit, less likely to smoke, used less alcohol and were more likely to have HPV than those who declined but did not differ by gender, cancer site, stage or time since diagnosis. Participants reported significantly better QoL and functioning and less severe symptoms than those who declined participation other than filling in the baseline questionnaire. Conclusions: Late symptoms are common in HNC survivors. A wide diversity of self-reported late effects was found in this trial with participants significantly less affected than non-participants. Implications for Cancer Survivors: Generalizable data on non-participation may aid in the interpretation of studies on HNC survivors in general and may have relevance for targeting recruitment and maintenance in rehabilitation and follow-up care, outside clinical trials.