BACKGROUND. Although discouraged, ad hoc chronic total occlusion (CTO) percutaneous coronary intervention (PCI) is occasionally performed. METHODS. We examined the clinical, angiographic characteristics, and procedural outcomes of patients who underwent ad hoc CTO PCI in the Prospective Global Registry for the Study of CTO Intervention (PROGRESS-CTO, NCT02061436). RESULTS. Of the 10,998 patients included in the registry, 899 (8.2%) underwent ad hoc CTO PCI. The incidence of ad hoc CTO PCI decreased from 18% in 2016 to 3% in 2022. Ad hoc CTO PCI patients had a lower prevalence of comorbidities and less complex angiographic characteristics demonstrated by lower J-CTO score (1.9±1.2 vs 2.4±1.3, P<.001). In these patients, PROGRESS-CTO major adverse cardiovascular events (MACE) (1.9±1.4 vs 2.5±1.7), mortality (1.2±1.0 vs 1.6±1.1), and perforation (1.5±1.2±2.2 vs 1.5) scores were lower (P<.001). Technical success was similar between the groups (86%). MACE were lower in the ad hoc CTO PCI group (.8% vs 2.0%, P=.009). Ad hoc CTO PCI was not associated with MACE after adjusting for potential confounders, odds ratio: .69 (95% confidence interval, .30-1.57). In patients with higher J-CTO scores, planned CTO PCI was associated with higher technical success (P<.001). CONCLUSION. Approximately 8% of CTO PCI procedures are performed ad hoc, usually in less complex lesions and patients with lower complication risk. While ad hoc CTO PCI might be appropriate for carefully selected cases, a staged approach is recommended for most CTO PCI. The Global Expert Consensus Document for CTO PCI discourages ad hoc CTO PCI to allow adequate time for procedural planning, preparation, and patient counseling.1 However, ad hoc CTO PCI is occasionally performed in daily practice,2 and such procedures have received limited study. We examined the frequency, clinical, angiographic, and procedural characteristics of patients who had ad hoc CTO PCI in a large international CTO PCI registry. Methods We examined the Prospective Global Registry for the Study of Chronic Total Occlusion Intervention (PROGRESS-CTO, NCT02061436) after stratifying patients based on CTO PCI planning into 2 groups: Ad hoc vs planned CTO PCI. PROGRESS-CTO includes patient-level data for CTO PCI procedures performed between 2012 and 2022 at experienced CTO PCI centers from the United States, Canada, Greece, Turkey, Egypt, Russia, and Lebanon.3,4 Study data were collected and managed using the REDCap (Research Electronic Data Capture) electronic data capture tools hosted at the Minneapolis Heart Institute Foundation.5,6 Definitions. CTOs were defined, according to the CTO Academic Research Consortium, as the absence of antegrade flow through the lesion with a presumed or documented duration of ≥3 months.7 Ad hoc CTO PCI was defined as PCI of a CTO that was discovered during the procedure (vs planned). Technical success was defined as the successful canalization of the CTO vessel with <30% residual stenosis and final Thrombolysis in Myocardial Infarction (TIMI) 3 flow.