Abstract Body (Do not enter title and authors here): Introduction: Postural orthostatic tachycardia syndrome (POTS) and Inappropriate sinus tachycardia (IST) are common manifestations of cardiovascular dysautonomia (CVAD) in patients with post-COVID-19 syndrome. Studies regarding differences between post-COVID-19 POTS and post-COVID-19 IST have been sparse and based on small patient series. Aims: To examine clinical differences between POTS and IST in patients with post-COVID-19 syndrome. Methods: A cross-sectional observational study based on a dataset of patients diagnosed with post-COVID-19 syndrome and POTS/IST, at Karolinska University Hospital, Stockholm in 2020-2023, was performed. Data was retrieved using patients’ medical records. ANOVA, chi-square tests and Fisher’s exact tests were used for analysis. Results: A total of 200 patients diagnosed with post-COVID POTS/IST (ICD-10 codes, I.498 + U.099) were included (female, 85%) and divided into a POTS-group (n=110) and IST-group (n=90). Sixty-one patients (31%) met the diagnostic criteria of both and were included in the IST-group. The mean ages were 38 years for the POTS-group and 42 years for the IST-group (p=0.027). Hypertension was more common within the IST-group (p<0.001). Of the entire study population, 73% developed their post-COVID-19 tachycardia after being infected by the SARS-CoV-2 virus in 2020. Non-cardiac symptoms were more common than cardiac symptoms and fatigue was the most frequently reported symptom amongst the entire study population, but both cognitive impairment (p=0.046) and nausea (p=0.020) were more commonly reported by the POTS-group. There was no significant difference regarding symptom burden according to Malmö POTS Score. Resting, average and maximum heart rate during 24-hour ECG monitoring were higher in the IST group compared to the POTS-group (89 vs. 78 bpm; 91 vs. 76 bpm; 157 vs.144; p<0.001). Ivabradine was the most frequently prescribed pharmacological agent within the study population, but Pyridostigmine was more commonly prescribed within the POTS-group (44% of the POTS-group vs. 31% of the IST-group, p=0.047). Conclusions: The diagnosis of POTS is more common than IST in patients suffering from post-COVID-19 syndrome, and there is a 31% overlap of patients diagnosed with both. There are some significant clinical differences between the groups, but both groups share the same symptom burden. The most commonly prescribed pharmacological treatment is Ivabradine, and Pyridostigmine is more frequently used for POTS.