Non-response can bias studies of disease conditions but its influence has rarely been evaluated due to limitations of available data on the non-respondents. Because of a detailed medical record review for eligibility, we were able to compare clinical as well as demographic characteristics of respondents and non-respondents in a population-based study of diabetic complications among Rochester, Minnesota residents. Non-respondents were older, less well educated, more likely to be widowed and more often retired. They were much more likely to have cardiovascular disease at baseline, but the prevalence of retinopathy, nephropathy and diabetic neuropathy was similar for respondents and non-respondents, who were also comparable with regard to type of diabetes and diabetic therapy. While these findings indicate that data from the Rochester Diabetic Neuropathy Study can probably be generalized to diabetic residents generally, they reemphasize the potential for non-response bias in epidemiologic studies of clinical conditions, especially cardiovascular disease.