Background The aim was to analyze the prevalence of dental treatments that were not performed in a dental care university referral center in the capital of Spain during the first wave of the COVID-19 pandemic. Methods This was a retrospective observational study based on the registry of medical records. Sex, age, nationality, and the type of treatment that was not performed in the service of the Integrated Adult Dental Clinic subject of the Dentistry degree at the Rey Juan Carlos University of Madrid were analyzed. Results A total of 392 medical records were analyzed. The prevalence of the treatments that were not performed was 58.67% (95% CI 53.74–63.44) of conservative treatments, 47.45% (95% CI 42.55–52.39) of periodontal treatments, 27.30% (95% CI 23.12–31.91) and 13.52% (95% CI 10.49–17.26) of clinical activities. The patients most affected by the absence of dental treatment ranged in age from 35 to 74 years. Age, sex, and nationality were not influential in not performing dental treatments. Conclusions The COVID-19 pandemic could have negatively influenced treatments, such as conservative and periodontal treatments, that increasing the risk of tooth loss in adults.