We sought to determine whether the exclusion of adults without landline telephones may bias estimates derived from health-related telephone surveys. METHODS: We took data from the 2004 and 2005 National Health Interview Survey and used logistic regression to compare the odds of behavioral risk factors and health care service use for adults with landline telephones to those for adults with only wireless telephones and adults without any telephone service. RESULTS: When interviewed, 7.2% of adults, including those who did and did not have wireless telephones, did not have landline telephones. Relative to adults with landline telephones, adults without landline telephones had greater odds of smoking and being uninsured, and they had lower odds of having diabetes, having a usual place for medical care, and having received an influenza vaccination in the past year. CONCLUSIONS: As people substitute wireless telephones for landline telephones, the percentage of adults without landline telephones has increased significantly but is still low, which minimizes the bias resulting from their exclusion from telephone surveys. Bias greater than 1 percentage point is expected only for estimates of health insurance, smoking, binge drinking, having a usual place for care, and receiving an influenza vaccination.