Age and gender are often suspected to be risk factors predisposing to ADRs. Therefore the data obtained by the Heidelberg Intensive Drug Monitoring System were analysed for possible correlations between these two variables and the incidence of ADRs. Based on the medical records comprising the time period between 1980 and 1987 information was available on 70,500 admissions to the Heidelberg University Hospital, Department of Medicine. Age, gender, number of prescriptions and ADRs were analysed. The percentage of patients affected by ADRs rises with advancing age; however the number of prescribed drugs also increases. When the incidence of ADRs in various age groups was analysed in relation to prescription data, no effect of age could be found. In contrast there was a linear correlation between the overall incidence of ADRs (independent of age) and the number of prescriptions per patient (per single admission). These results clearly document that age does not seem to be relevant to the incidence of ADRs, but that the risk is related to the number of drugs prescribed to a particular patient.