Results obtained from self-reported health data may be biased if those being surveyed respond differently based on health status. This study was conducted to investigate if health, as measured by health care use preceding invitation, influenced response to invitation to a 21-year prospective study, the Millennium Cohort Study. Inpatient and outpatient diagnoses were identified among more than 68,000 people during a one-year period prior to invitation to enroll. Multivariable logistic regression defined how diagnoses were associated with response. Days spent hospitalized or in outpatient care were also compared between responders and nonresponders. Adjusted odds of response to the questionnaire were similar over a diverse range of inpatient and outpatient diagnostic categories during the year prior to enrollment. The number of days hospitalized or accessing outpatient care was very similar between responders and nonresponders. Study findings demonstrate that, although there are some small differences between responders and nonresponders, prior health care use did not affect response to the Millennium Cohort Study, and it is unlikely that future study findings will be biased by differential response due to health status prior to enrollment invitation.