Errors involving diagnostic test ordering are common in outpatient care, even in highly computerized and integrated health care systems. Failure to follow up on test results has been linked to missed and delayed diagnoses and significant patient harm. This study used a voluntary error reporting system based in eight outpatient family medicine clinics to examine the types and severity of errors in the testing process. Errors were documented at each step of the process. Ranging from incorrect test ordering to failure to provide test results to clinicians and patients, these errors frequently resulted in failure to provide timely patient care. A companion article evaluated factors that reduced harm to patients from testing errors.