Background: To reduce the burden caused by emergency department (ED) visits and ambulance use, after-hours house call (AHHC) medical services have been implemented. Examining the pre-and post-home visit behaviors of those who use AHHC medical services, stratified by age and illness severity, may help determine the populations these services should target effectively to reduce ED visits and ambulance use.Methods: This questionnaire-based study used data from anonymized medical records and internet-based questionnaires completed by patients who used AHHC medical services in Tokyo, Japan, between January 1 and December 31, 2019. The questionnaire consisted of two questions: what action would the patient have taken in the absence of AHHC services, and what action was taken within three days following the use of the AHHC services.Results: Of the 15,787 patients who used AHHC medical services during the study period, 2,128 completed the questionnaire (13.5% response rate). Individuals aged <15 years and 16–64 years were the most common users of AHHC services (≤15 years, 71.4%; 16–64 years, 26.8%). Before using the AHHC service, the majority of patients in each patient age group would have visited the ED if the AHHC service had not been available (≤15 years, 47.8%; 16–64 years, 42.8%; ≥65 years, 43.6%), and the proportion of patients originally planning to call an ambulance was higher among those in higher age groups (≤15 years, 1.1%; 16–64 years, 6.0%; ≥65 years, 20.5%). After using AHHC services, the majority of patients (68.1%) did not visit a hospital within three days, but the proportion of patients who visited an ED and called an ambulance within that time increased as their illness severity increased.Conclusions: Increasing AHHC medical services awareness among older adults and encouraging repeated use of AHHC services for patients with severe illnesses may help reduce ED visits and ambulance use.