BACKGROUND Questionnaire surveys are important for surveying the health and disease behaviour of the population, but recent years have seen a fall in participation. Our study tested whether incentives can increase participation in these surveys.MATERIAL AND METHOD We sent a questionnaire on risk factors for colorectal cancer (height, weight, smoking, self-reported diagnoses, family medical history) to non-screened participants in a randomised colonoscopy screening study for colorectal cancer: participants who were invited but did not attend for colonoscopy examination (screening-invited) and persons who were not offered colonoscopy (control group). The persons were randomised to three groups: no financial incentive, lottery scratch cards included with the form, or a prize draw for a tablet computer when they responded to the form. We followed up all the incentive groups with telephone reminder calls, and before the prize draw for the tablet computer.RESULTS Altogether 3 705 of 6 795 persons (54.5 %) responded to the questionnaire; 43.5 % of those invited for screening and 65.6 % of the control group (p < 0.001). The proportion that answered was not influenced by incentives, either among those invited for screening (42.4 % in the non-prize group, 45.5 % in the lottery scratch card group and 42.6 % in the prize draw group; p = 0.24), or in the control group (65.6 % in the non-prize group, 66.4 % in the lottery scratch card group and 64.7 % in the prize draw group; p = 0.69). Prior to reminder calls, 39.2 % responded. A further 15.3 % responded following telephone reminder calls (14.1 % of the screening-invited and 16.5 % of the control group; p < 0.001).INTERPRETATION Incentives did not increase participation in this medical questionnaire survey. Use of telephone reminder calls and telephone interviews increased participation, but whether this is more effective than other methods requires further study.