Cancer care has been the setting for seminal, practice-changing errors. This retrospective study aimed to identify adverse events in cancer care through medical record review, using a random sample of breast, colorectal, and lung cancer cases from 2012. As with prior studies, physician investigators determined preventability and extent of harm. Over a third of patients experienced an adverse event, and about 32% of adverse events were deemed preventable. Most adverse events occurred in the inpatient setting. Adverse events included medication errors and hospital-acquired conditions, such as pressure ulcers and falls. The authors conclude that patient safety remains an important consideration for cancer care that merits further research and improvement efforts.