He also noted that previous financial regulations were "half a century or more old," and that the financial crisis emerged from an industry that was poorly regulated and not well understood.
Lew's comments came during an interview that also touched on immigration reform, revelations that the Internal Revenue Service had singled out conservative groups for extra scrutiny and the broader economic recovery.
Some legislators worry that more needs to be done.
Sens. Sherrod Brown (D-Ohio) and David Vitter (R-La.) introduced legislation in April to tighten controls on big banks. The bill would increase the amount of money most banks need to hold in reserve, limit financial institutions' ability to transfer money between affiliates and reduce burdens for small institutions.