This single-center, case-control study documents a relative increase in methicillin resistance among 48 cases of Staphylococcus aureus-associated postpartum mastitis during 1998-2005. Of 21 cases with methicillin resistance, 17 (81%) occurred in 2005. Twenty (95%) isolates contained the Staphylococcus cassette chromosome mec type IV gene; this suggests that the increase is due to community-acquired methicillin-resistant S. aureus.