We examined the effects of the COVID-19 pandemic on the screening, diagnosis and treatment of breast cancer in Hungary based on administrative data until June 2021, covering three pandemic waves. After correcting for trend and seasonality, the number of mammography examinations decreased by 68% in 2020q2, was around its usual level in 2020q3 and was reduced by 20–35% throughout 2020q4-2021q2. The reduction was caused by a combination of supply-side (temporary suspensions of screening) and demand-side (lower screening participation during the pandemic waves) factors. The number of new breast cancer diagnoses and mastectomy surgeries responded with a lag, and were below their usual level by 15-30% in all quarters between 2020q2 and 2021q2, apart from 2020q4, when there was no significant difference. Using a regression discontinuity framework, we found that the partial mastectomy rate (indicative of early diagnosis) dropped more substantially in 2020q2 in the 61–65 years old age group that was just below the age cut-off of organized screening than in the 66–70 years old age group, and this difference was partially offset in 2021q1. We suggest that policymakers need to motivate the target population (by providing both information and incentives) to catch up on missed screenings.