INTRODUCTION: The first hours after the admission of patients, and proper medical care is administered in the emergency department [ED], are of decisive importance in protecting them from unexpected death. Medical staff and researchers are not consistent in the period to follow up on deaths after admission to the emergency department and they analyze arbitrarily different time intervals without any justification for the chosen period. In this study, we will conduct an epidemiological data analysis to determine the range of the most dangerous (elevated) risk (hazard) of death for patients within one month of observation from an ED admission using modern survival modeling and software. MATERIAL AND METHODS: Epidemiological data analysis of the three most common non-traumatic diseases (neoplasms, circulatory, and endocrine) was carried out in this study. Using the 2016–2019 sample of 14,904 first-visit ED patients at the Multi-Specialistic Hospital in Gorzów Wielkopolski, Poland, we determined the range of the most dangerous (elevated) risk (hazard) of death within one month of observation, based on a Royston–Parmar (RP) regression with spline functions (assuming non-constant hazard over time). RESULTS: The results show that in the three most common non-traumatic diseases (neoplasms, circulatory, and endocrine) for the first 72 hours, patients should be under special supervision of medical personnel to avoid an excess of unexpected deaths. Moreover, within a month from ED admission, the hazard ratio [HR] of death was almost half as high [HR = 1.47, 95% confidence interval (CI) = 1.07 to 2.02] in diagnosed circulatory patients and over twice as high (HR = 2.25, 95% CI = 1.58 to 3.20) in neoplastic diseases as compared to reference endocrine patients. Moreover, the estimated RP hazards (probabilities of death) increased until the third day after admission, reaching 1.0% (95% CI = 0.8% to 1.4%) of endocrine patients, 1.5% (95% CI = 1.3% to 1.6%) of circulatory patients, and 2.2% (95% CI = 1.8% to 2.6%) for neoplasms, and then dropped radically with the time of observation. CONCLUSIONS: In view of the care of patients in the three most non-traumatic clinical diagnoses (endocrine diseases, circulatory diseases, and neoplasms), special attention should be paid to the first three days after admission to the ED (after this time, in the first month of observation, the risk of death of these patients decreases significantly).