AbstractPurposeIn epidemiological studies, non‐response may introduce bias and limit generalizability. In genetic pharmacoepidemiological research, collection of DNA might be a major reason for non‐response. We determined reasons for non‐response and compared characteristics of non‐responders and responders in a pharmacogenetic case‐control study.MethodsMyocardial infarction (MI) cases and controls, who were antihypertensive drug users, were recruited through community pharmacies that participate in the Pharmaco‐Morbidity‐Record‐Linkage‐System (PHARMO). The PHARMO database comprises drug dispensing histories of about 2 000 000 subjects from a representative sample of Dutch community pharmacies linked to the national registry of hospital discharges. Independent samples t‐test and ANOVA‐statistics were used to analyse the differences in continuous variables between responders and non‐responders. χ 2 statistics and logistic regression were used to compare categorical variables.ResultsWe approached 1871 cases and 14 102 controls of whom 794 MI cases (42.4%) and 4997 controls (35.4%) responded. We could not approach 2194 patients of whom 63.1% had died and 31.2% moved to another pharmacy. Main reasons for non‐response were health problems or hospital stays (16.2%, OR 1.47; 95%CI: 1.00–2.16). Other reasons were old age or dementia (16.9%, OR 1.82; 95%CI: 1.24–2.65). Only a small percentage (1.1%, OR 1.43; 95%CI: 0.41–5.03) mentioned DNA sampling as a reason. About 30% of the non‐responders did not give a reason. Women were statistically significantly (p < 0.0005) less willing to participate than men (38.8% versus 31.3%). An association with age was also found (mean age 64.6 versus 66.5 yrs) (p < 0.0005).ConclusionIn a pharmacogenetic case‐control study fear for genetic screening was not a major reported reason for non‐response. Females were less willing to participate than males. Copyright © 2009 John Wiley & Sons, Ltd.