The incidence and causes of iatrogenic illness in a department of internal medicine were studied prospectively in 1,176 patients admitted from 1 January 1986 to 31 December 1986. A total of 295 patients (25.1%) developed 367 episodes of iatrogenic illness. Phlebitis occurred most frequently (75.2% of all iatrogenic events), followed by drug reactions (10.7%), contusion (4.6%), and urinary tract infections (1.4%). Nineteen patients developed life-threatening events (in 2 it was the cause of death). Etiologic agents included intravenous catheter (79% of all adverse reactions), drugs (9.5%), falls from bed (5.4%), diagnostic procedures (3.3%), and urinary catheterization (1.6%). Risk factors associated with iatrogenic illness were hospital stay longer than 12 days, female sex, poor general medical status on admission, intravenous catheterization, and intravenously administered antibiotics and anticoagulants. We conclude that in our hospital (a) 25% of the inpatient admissions to the general medicine service resulted in iatrogenic illness, (b) most iatrogenic illnesses were not severe (phlebitis), but 2 of 19 patients with life-threatening events died, and (c) the probability of developing iatrogenic illness generally depended on long hospital stay, poor general status at admission, and the use of both intravenous catheters and medication. In our patients, a reduction of hospital stay and a rational limitation of these procedures may diminish the rate of complications.