We conducted a HUS to help inform establishment of a sentinel surveillance system for pneumonia and influenza-like illness in Santa Rosa, Guatemala, and found that private clinics are the single most important source of healthcare for pneumonia both in children <5 years old and older persons. For more mild ILI, both children <5 years old and persons aged five years or older are more likely to consult government clinics than other sources of health care. These findings suggest that in Santa Rosa, sentinel surveillance for pneumonia in government hospitals will significantly underestimate the burden of disease, by up to 75% for children <5 years old and 88% for persons aged five years and older. Government healthcare clinics will underestimate the number of cases of ILI by about 59% for children <5 years old and 64% for persons five years and older.