<sec id="st1"> <title>SETTING</title> The household and non-household contacts of patients with tuberculosis (TB) face varying degrees of risk of infection by Mycobacterium tuberculosis. </sec> <sec id="st2"> <title>OBJECTIVE</title> To quantify new infection and to determine the risk factors associated with new infection among named contacts in San Francisco, CA, USA. </sec> <sec id="st3"> <title>DESIGN</title> We performed a cohort study in patients with culture-positive pulmonary TB. We analyzed patient, contact, environmental and bacterial characteristics. </sec> <sec id="st4"> <title>RESULTS</title> Of the 2422 contacts named by 256 patients, 149 (6.2%) had new infection due to recent transmission from 79 (30.9%) patients. Of the 149 new infections, 87 (58.4%) occurred among household contacts and 62 (41.6%) among non-household contacts. Numerous acid-fast bacilli in sputum (odds ratio [OR] 2.64, 95%CI 1.32-5.25) and contacts being named by more than one patient (OR 2.90, 95%CI 1.23-6.85) were associated with new infection among household contacts. Being older than 50 years (OR 1.93, 95%CI 1.09-3.41) and an Asian/Pacific Islander (OR 3.09, 95%CI 1.50-6.37) were associated with new infection among non-household contacts. </sec> <sec id="st5"> <title>CONCLUSIONS</title> Fewer than one third of patients caused new infection to his/her contacts. A substantial proportion of transmission resulting in new infection occurred outside of the household. The risk factors for infection among household and non-household contacts are different and should be considered when prioritizing control interventions. </sec>.