There are few data on the safety of length of stay in uncomplicated ST-segment elevation myocardial infarction. We studied trends in hospital stay and the safety of short (≤ 3 days) vs long hospital stay in Spain. Using data from the Minimum Basic Data Set, we identified patients with uncomplicated ST-segment elevation myocardial infarction who underwent primary percutaneous coronary intervention and who were discharged alive between 2003 and 2015. The mean length of stay was adjusted by multilevel Poisson regression with mixed effects. The effect of short length of stay on 30-day readmission for cardiac diseases was evaluated in episodes from 2012 to 2014 by propensity score matching and multilevel logistic regression. We also compared risk-standardized readmissions for cardiac diseases and mortality rates. The adjusted length of stay decreased significantly (incidence rate ratio <1; P <.001) for each year after 2003. Short length of stay was not an independent predictor of 30-day readmission (OR, 1.10; 95%CI, 0.92-1.32) or mortality (OR, 1.94; 95%CI, 0.93-14.03). After propensity score matching, no significant differences were observed between short and long hospital stay (OR, 1.26; 95%CI, 0.98-1.62; and OR, 1.50; 95%CI, 0.48-5.13), respectively. These results were confirmed by comparisons between risk-standardized readmissions for cardiac disease and mortality rates, except for the 30-day mortality rate, which was significantly higher, although probably without clinical significance, in short hospital stays (0.103% vs 0.109%; P <.001). In Spain, hospital stay ≤ 3 days significantly increased from 2003 to 2015 and seems a safe option in patients with uncomplicated ST-segment elevation myocardial infarction.