Background: Clinical spectrum of COVID-19 has been unclear, especially with regard to the presence of pneumonia. We aimed to present clinical course of all laboratory-confirmed adult COVID-19 patients and to identify potential predicting factors of pneumonia. Methods: We conducted a retrospective study among adult patients with confirmed COVID-19 who were hospitalized at Bamrasnaradura Infectious Diseases Institute, Thailand, regardless of their disease severity, between January 8 and April 16, 2020. We described the full picture of COVID-19, defined definite outcomes and evaluated factors associated with pneumonia. Results: One-hundred-and-ninety-three patients were included. The median (IQR) age was 37.0 (29.0-53.0) years, and 58.5% were male. Of whom, 189 (97.9%) recovered and 4 (2.1%) died. More than half (56%) of the patients were mild, 22% were moderate, 14% were severe, and 3% were critically ill. Asymptomatic infection was found in 5%. The overall incidence of pneumonia was 39%. Bilateral was more prevalent than unilateral pneumonia (65% vs. 35%). Increasing age (OR 2.60 for every 10-year increase from 30 years old; 95% CI, 1.68 to 3.97; p<0.001), obesity (OR 9.17; 95% CI, 2.11 to 39.89; p=0.003), and higher temperature at presentation (OR 4.66 per one-degree Celsius increase from 37.2 degree Celsius; 95% CI, 2.32 to 9.34; p<0.001) were potential predicting factors of COVID-19 pneumonia. Severe cases had a longer viral RNA shedding duration than the non-severe cases. The longest observed duration of viral RNA shedding was 45 days. Conclusion: Across different disease severities, most patients with COVID-19 in Thailand had a good prognosis. COVID-19 pneumonia was found in one-third of the hospitalized patients. Potential predicting factors included old age, obesity, fever at presentation.