According to test results obtained by the Denver Post, it is the highest number of homes affected in 12 years.
Lead can cause brain and nerve damage, especially in children.
Colorado health officials say doctors are reporting high blood levels of lead in Denver and other counties around Colorado.
This month, all 1.3 million metro residents served by Denver Water must be notified.
Precautions include letting faucets run a while to flush out lead, using a water filter and having household water tested.