Background The aging population is an increasing concern in Western hospital systems. The aim of this study was to describe the main characteristics and hospitalization patterns in very elderly inpatients (≥ 85 years) in Spain from 2000 to 2015.Methods Retrospective observational study analyzing data from the minimum basic data set, an administrative registry recording each hospital discharge in Spain since 1997. We collected administrative, economic and clinical data for all discharges between 2000 and 2015 in patients aged 85 years and older, reporting results in three age groups and four time periods to assess differences and compare trends.Results There were 4,387,326 admissions in very elderly patients in Spain from 2000 to 2015, representing 5.32% of total admissions in 2000–2003 and 10.42% in 2012–2015. The pace of growth was faster in older age groups, with an annual percentage increase of 6% in patients aged 85–89 years, 7.79% in those aged 90–94 years, and 8.06% in those aged 95 and older. The proportion of men also rose (37.3% to 39.7%, p<0.001), and they had a higher risk of hospitalization than women (385 discharges/1000 men versus 280 discharges/1000 women in 2012–2015).Mortality decreased from 14.64% in 2000–2003 to 13.83% in 2012–2015 (p<0.001), and mean length of stay from 9.98 days in 2000–2003 to 8.34 days in 2012–2015. Costs per hospital stay increased from 2000 to 2011, from EUR 4611 in 2000–2003 to EUR 5212 in 2008–2011, before dropping to EUR 4824 in 2012–2015. The 10 most frequent discharge diagnoses in the period 2000-2003 were: femoral neck fracture (8.07%), heart failure (7.84%), neoplasms (7.65%), ischemic encephalopathy (6.97%), pneumonia (6.36%), chronic obstructive pulmonary disease (4.23%), ischemic cardiomyopathy (4.2%), other respiratory diseases (3.87%), other alterations of urethra and the urinary tract (3.08%), and cholelithiasis (3.07%). Conclusions The very elderly population is growing in Spanish hospitals, and within this group, patients are getting older and more frequently male. M ean length of stay, cost of stay, and mortality are decreasing. Decompensation of chronic diseases, neoplasms and infections are the most common causes of admission.