BACKGROUND It's obvious that the current medicine practice generates iatrogenesis . However, we are unaware about its magnitude and severity, specially in Spain, where this item has been scarcely studied. METHODS All the patients admitted to the Department of Internal Medicine were prospectively studied during a 13 months period, selecting among the patients those fulfilling criteria for an iatrogenic problem both of pharmacological origin and not pharmacological. The kind of iatrogenesis, its severity, related mortality, gravity of the basic illness, affected organ and avoidability of the iatrogenesis were evaluated. Infusion phlebitis were recorded only during 6 months. RESULTS Iatrogenic pathology was found in 228 cases over 1.549 patients admissions, accounting for 14.7% of incidence. Iatrogenic pathology was the reason for admission in 65 cases. The average stay was significantly increased in patients with iatrogenic pathology (p < 0.01). Adverse reactions to drugs accounted for 62% of the total account with 141 cases. The non-steroids antiinflammatory (NSA) drugs were the most frequently troublesome pharmacological agents. The GI tract was the more affected system (84 cases). Infusion phlebitis are not included in the total account of cases. CONCLUSIONS a) iatrogenic pathology is an illness of very high incidence in our surroundings; b) NSA is a group of drugs generating frequently adverse reactions in off-hospital environment; c) GI hemorrhage is an iatrogenic illness accounting for high percentage of cases; d) many of the iatrogenic events can be catalogued as avoidable and with more accurate attention to some factors the more of them could be prevented.