To determine how long people shed virus after the onset of mumps, we used logistic regression modeling to analyze data from the 2006 outbreak of mumps in Iowa. Our model establishes that the probability of mumps virus shedding decreases rapidly after the onset of symptoms. However, we estimate that 8%-15% of patients will still be shedding the virus 5 days after the onset of symptoms and, thus, may still be contagious during this period.