On February 3, 1946, medical literature recorded what is generally considered the first well-documented clinical diagnosis of acute radiation sickness (ARS) in a human being. The case arose during early experiments and clinical observations following the rapid expansion of X‑ray and radiation technology in the first half of the 20th century and the wartime development of nuclear weapons and reactors. Although radiation effects had been suspected and reported earlier—particularly skin burns and chronic changes among radiologists and radium dial painters—this 1946 report is notable for describing a constellation of systemic symptoms attributed directly to a single, significant exposure and for treating the condition in the language of “radiation sickness.” Background Before 1946, clinicians had observed localized injuries from ionizing radiation (erythema, burns, ulceration) and chronic conditions (cancers, necrosis) among workers who handled radium or endured repeated X‑ray exposure. However, the concept of an acute, whole‑body syndrome caused by a large, single or short-term exposure—now called acute radiation syndrome—was not yet systematically described in clinical terms. The expansion of large-scale radiation work during World War II, and the subsequent medical scrutiny of radiation incidents, prompted closer attention to acute systemic effects. The 1946 report The February 3, 1946 account documented a patient with a clear history of significant exposure to ionizing radiation who developed a sequence of symptoms now recognized as hallmark features of ARS: nausea and vomiting within hours to days, diarrhea, fever, progressive loss of white blood cells (leukopenia), bleeding, and infections. The clinicians who managed the case correlated the timing and progression of these systemic signs with a documented exposure event and used emerging radiation biology knowledge to interpret the findings. Their clinical description emphasized both the hematologic collapse and the constitutional symptoms that distinguished ARS from localized radiation injury. Significance and context This report contributed to formalizing medical recognition of ARS and helped spur development of diagnostic criteria, supportive care practices, and occupational safety standards. In the years immediately following 1946, both military and civilian medical communities synthesized wartime and peacetime experience—particularly from atomic bombings and reactor incidents—to better understand dose–response relationships, latency of effects, and therapeutic approaches such as transfusions, antibiotics, and hematopoietic support. Uncertainties and earlier reports It is important to note that earlier historical records document radiation-related illness of various kinds: chronic radium poisoning among dial painters in the 1920s and 1930s, severe X‑ray burns in early radiologists, and scattered case reports of systemic illness following exposure. Some historians and clinicians debate which single report should be labeled “the first” documented case of radiation sickness, because definitions evolved over time and earlier clinicians sometimes described similar syndromes without the modern terminology. The February 3, 1946 report is widely cited for its clear clinical framing of an acute, whole‑body radiation syndrome, but it sits within a continuum of growing medical awareness rather than as an isolated discovery. Aftermath Recognition of acute radiation sickness led to improved occupational regulations, dosimetry practices, protective equipment, and emergency protocols. Over subsequent decades, international bodies and national health agencies developed formal case definitions, triage strategies, and treatment guidelines that remain the basis for contemporary medical responses to significant radiation exposures. Sources and further reading This summary is based on historical medical literature and reviews of radiation medicine history. Where precise attribution of an earliest single case varies among historians, I note that the 1946 clinical report is frequently referenced in medical histories as the first clearly documented diagnosis of acute radiation sickness as a systemic clinical entity.