To analyze the influence of recent changes in Minnesota statutes that generally require prior authorization for use of medical records for research from patients who received medical care after Jan. 1, 1997. In this Mayo Clinic Institutional Review Board-approved study, we obtained a stratified random sample of patients encountered at Mayo Clinic Rochester during the period 1994 through 1996 and estimated the proportion willing to provide the general authorization. On the basis of data from administrative files, we then compared demographic, diagnostic, and utilization characteristics for patients who provided authorization and those who did not. Overall, 3.2% (95% confidence interval, 2.4 to 4.0%) of the study subjects declined authorization. If patients not responding to requests for authorization were also considered to have refused, the overall refusal rate would be 20.7% (95% confidence interval, 18.5 to 22.9%). Women were somewhat more likely to refuse authorization than were men (4.0% versus 2.4%; P = 0.067), and patients younger than 60 years were more likely to refuse than were older patients (5.4% versus 1.2%; P<0.001). Patients residing more than 120 miles from Rochester were much less likely to decline authorization than were local residents (2.1% versus 5.8%; P = 0.001). Patients with prior diagnoses that might be considered more sensitive such as mental disorders, infectious diseases, and reproductive problems also were more likely to refuse authorization. These data demonstrate that laws requiring written authorization for research use of the medical record could result in substantial biases in etiologic and outcome studies, the direction and magnitude of which may vary from topic to topic. Clinicians should be prepared to enter the discussion to help inform patients and legislators of the potential hazards of laws that restrict access to medical records for research purposes.