The beginning of the COVID-19 pandemic immediately changed how patients sought healthcare. This study analyzed the change frequency of diagnoses made in 2019 compared to 2020 in one region of Spain. On average, the number of diagnoses declined 31% from 2019 to 2020, with cancer diagnoses declining by nearly 50%. As COVID-19 cases continue to decrease in many areas in 2021, the authors recommend local, regional, and national public health leaders prioritize plans to target under-diagnosed conditions.