Diagnostic errors can result in significant morbidity and mortality. This large cohort study reviewed the health records of 2,428 adult inpatients who were transferred to the intensive care unit (ICU) and/or died in the hospital to estimate frequency, cause, and harms of diagnostic errors. Nearly a quarter (23.0%) of patients in the cohort experienced a diagnostic error, and 6.6% of patients who died had a diagnostic error. Delays in ordering and interpreting tests and problems with clinical assessment were the most common contributing factors resulting in transfer to ICU and/or death.