The Syndrome of Inappropriate Antidiuresis (SIADH) is characterized by a low serum sodium, reduced serum osmolality, relatively high urine osmolality and urine sodium together with a clinically euvolemic patient in absence of diuretic medication [
1]. SIADH has been described to be associated with a multitude of conditions and medications with diseases of the lung being quite prominent [
1]. Only recently, SIADH was also described in patients infected with the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) [
2,
3]. To this date, intensive vaccination programs have started world-wide in order to fight the pandemic [
4]. In our country at this time, only Spikevax (mRNA-1273, Moderna) and Comirnaty (BNT162b2, Pfizer-BioNtech) were approved by our local drug administration. They both contain messenger ribonucleic acid (mRNA) coding for spike-proteins of the SARS-CoV-2 virus. The mRNA is transported into hosting cells in lipid nanoparticles, where the proteins are synthesized and presented on the cell membrane [
5,
6]. This leads to an immune reaction against the presented antigens, which results in a T- and B-Cell activation and consecutively in a humoral and cellular immune response [
5,
6]. The most common side effect is a solicited local reaction [
5,
6]. Furthermore, common side effects reach from fever, headache and fatigue to lymphadenopathy, myalgia, arthralgia, nausea/vomiting and chills. Unfortunately, there are also rare cases of anaphylactic shock after receiving either of the mentioned vaccines. Recently, a case series of Myocarditis and Pericarditis associated with COVID-19 vaccination was published [
7]. Since vaccination programs started, a lot of case reports were published describing a multitude of pathologies possibly related to the vaccination. They are reaching from thyreoiditis, over Guillan-Barré syndrome, to stroke-mimics and uveitis, to only mention a few [
8‐
11]. We describe the first case of a patient with newly diagnosed and symptomatic SIADH after receiving the second COVID-19 vaccination not explained otherwise.