To evaluate the prevalence of colonization among patients with community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infection and their household contacts.

Prospective, observational laboratory study of nasal colonization among patients and their household members from September 15, 2004, to February 20, 2006.

A 600-bed, urban, academic medical center.

Fifty-one patients who presented with CA-MRSA infections and 49 household members had cultures of nasal swab specimens performed.

Skin and soft-tissue infections were seen in 50 patients (98%) and 2 household members. Twenty-one (41%) of 51 patients and 10 (20%) of 49 household members were colonized with MRSA. An additional 5 patients (10%) and 12 household members (24%) were colonized with methicillin-susceptible Staphylococcus aureus. Most MRSA isolates (95%; infective and colonizing) carried the staphylococcal cassette chromosome mec type IV complex, and 67% represented a single clone, identical to USA 300. Of the colonized household members, 5 had isolates related to the patients' infective isolate.

The frequency of CA-MRSA colonization among household members of patients with CA-MRSA infections is higher than rates reported among the general population. Among colonized household members, only half of the MRSA strains were related to the patients' infective isolate. Within the same household, multiple strains of CA-MRSA may be present.