Hospitalisations caused by adverse drug reactions (ADR): a meta-analysis of observational studies. 
To establish the percentage hospital admission related to adverse drug reactions (ADRs) from the data available in the literature.  Literature search in the Medline database, meta-analysis.  From the literature it is revealed that a considerable part of all hospital admissions are related to adverse drug reactions.  However, these data are not homogenous, i.e. larger studies display a lower percentage of ADR related hospital admission, while smaller studies display a higher percentage.  Subgroup analysis showed that for elderly people the odds of being hospitalised by ADR related problems is 4 times higher than for younger ones (16.6% vs. 4.1%).  A considerable part of these hospitalisations can be prevented.  Subgroup analysis revealed that in the elderly up to 88% of the ADR related hospitalisations are preventable; for the non-elderly this is only 24%.  Comparatively more elderly people are hospitalised than younger ones.  Combining these findings, twice as much elderly people are hospitalised by ADR related problems than non-elderly, while preventability of ADR related hospitalisation might yield 7 times more people in the elderly than in the non-elderly.  The estimation of the costs of ADR related hospitalisations in the Health Care system in The Netherlands is discussed.  Many elderly people are hospitalised by ADR related problems; an important part of these hospitalisations can be avoided.