BACKGROUND: To identify factors associated with hospital readmission, especially those potentially avoidable. This information could be useful to reduce the incidence of hospital readmissions. METHODS: A paired (1:1) case-control study nested into the cohort of first admissions at the Granada University Hospital, Spain, in 1990. All patients readmitted at the hospital within 3 years after release from the index-admission by the same diagnostic or complications of it comprised the case group. For each case, a control patient was matched for both primary diagnosis on admission and index-admission date. Information on risk factors associated with hospital readmission was obtained retrospectively from medical records. The relation between these risk factors and readmission was estimated from odds ratio both crude and adjusted using conditional logistic regression analysis. For the readmitted subsample, multiple linear regression models were applied to identify factors associated to the length of time between the index episode and the first readmission. RESULTS: Variables with consistent positive associations with readmission include male sex (odds ratio = 2.86, 95% confidence interval = 1.37-5.88), widowed or single status (2.66, 10.7-6.59) and severity at index admission (3.20, 1.57-6.51). Factors related to quality of health care did not influence readmission risk. CONCLUSIONS: Factors depending of the patient seem to be the most important variables associated to the incidence of hospital readmission.