Overdiagnosis is an emerging safety concern due to its potential to result in physical, financial, and emotional harm. Researchers surveyed 533 primary care practitioners (physicians, nurse practitioners, and physician assistants) and asked them to estimate the probability of disease for common conditions (pneumonia, cardiac ischemia, breast cancer screening, and urinary tract infection) and the association of positive and negative test results with disease probability. Findings indicate that significant overestimation of disease among all participating practitioners – likely due to overestimates of pretest probability – may contribute to overdiagnosis and overuse.