BACKGROUND Studies of hospitalized patients identify safety as a significant problem, but few data are available regarding injuries occurring after discharge. Patients may be vulnerable during this transition period. OBJECTIVE To describe the incidence, severity, preventability, and “ameliorability” of adverse events affecting patients after discharge from the hospital and to develop strategies for improving patient safety during this interval. DESIGN Prospective cohort study. SETTING A tertiary care academic hospital. PATIENTS 400 consecutive patients discharged home from the general medical service. MEASUREMENTS The three main outcomes were adverse events, defined as injuries occurring as a result of medical management; preventable adverse events, defined as adverse events judged to have been caused by an error; and ameliorable adverse events, defined as adverse events whose severity could have been decreased. Posthospital course was determined by performing a medical record review and a structured telephone interview approximately 3 weeks after each patient’s discharge. Outcomes were determined by independent physician reviews. RESULTS Seventy-six patients had adverse events after discharge (19% [95% CI, 15% to 23%]). Of these, 23 had preventable adverse events (6% [CI, 4% to 9%]) and 24 had ameliorable adverse events (6% [CI, 4% to 9%]). Three percent of injuries were serious laboratory abnormalities, 65% were symptoms, 30% were symptoms associated with a nonpermanent disability, and 3% were permanent disabilities. Adverse drug events were the most common type of adverse event (66% [CI, 55% to 76%]), followed by procedure-related injuries (17% [CI, 8% to 26%]). Of the 25 adverse events resulting in at least a nonpermanent disability, 12 were preventable (48% [CI, 28% to 68%]) and 6 were ameliorable (24% [CI, 7% to 41%]). CONCLUSIONS Adverse events occurred frequently in the peridischarge period, and many could potentially have been prevented or ameliorated with simple strategies.