Context: In May 1997, the US Nuclear Regulatory Commission (NRC) revised its patient release regulations, allowing for outpatient administration of larger activities of sodium iodide 131I than previously permitted. Objective: To measure the radiation exposure to household members from patients receiving outpatient 131I therapy for thyroid carcinoma in accordance with the new regulations. Design: Consecutive case series from October 1998 to June 1999. Settings and Patients: Thirty patients who received outpatient 131I therapy following thyroidectomy for differentiated thyroid carcinoma were enrolled, along with their 65 household members and 17 household pets. Main Outcome Measure: Radiation exposure to household members and 4 rooms in each home, as monitored with dosimeters for 10 days following 131I administration. Results: The patients received 131I doses ranging from 2.8 to 5.6 GBq (mean, 4.3 GBq). The radiation dose to 65 household members ranged from 0.01 mSv to 1.09 mSv (mean, 0.24 mSv). The dose to 17 household pets ranged from 0.02 mSv to 1.11 mSv (mean, 0.37 mSv). The mean dose to the 4 rooms ranged from 0.17 mSv (kitchen) to 0.58 mSv (bedroom). Conclusion: In our study, 131I doses to household members of patients receiving outpatient 131I therapy were well below the limit (5.0 mSv) mandated by current NRC regulations.