The cost of inpatient percutaneous coronary interventions (PCI) procedure is related to length of stay (LOS). It is unknown, how LOS may be associated with readmission rates and costs of index PCI and readmissions in elective PCI. This study aims to evaluate rates, predictors, causes, and costs associated with 30-day unplanned readmissions according to lLOS in patients, who underwent elective PCI. We included patients in the Nationwide Readmission Database, who were admitted to hospital from 2010 to 2014, who underwent uncomplicated elective PCI. LOS was defined as 0, 1, 2, and ≥3 days. A total of 324,345 patients were included in the analysis and the 30-day unplanned readmission was 4.75%, 4.67%, 6.44%, and 9.42% in the LOS groups 0, 1, 2, and ≥3 days, respectively. Prolonged LOS was associated with increased average total 30-day cost (index and readmission cost, 0 days $15,063, 1 day $14,693, 2 days $18,136, and ≥3 days $24,336). Compared with 0 days, the odds of readmissions were greater for 2 days (odds ratio 1.41, 95% confidence interval 1.07 to 1.87, p = 0.016) and ≥3 days (odds ratio 1.70, 95% confidence interval 1.28 to 2.24, p <0.001). Comorbidities were strong predictors of LOS and noncardiac causes, account for more than half of all causes for readmission. Longer LOS was associated with reduced incidence of readmissions for noncardiac causes such as noncardiac chest pain, but a greater rate of readmissions for heart failure. In conclusion, shorter length of stay was associated with reduced healthcare costs in elective PCI.