Aim: There is growing interest in the importance of low serum total IgE level in the recent years due to data showing its relation to immune system dysfunction syndromes and susceptibility to malignant neoplastic diseases. Material and Method: Within the period 01/01/2013 and 31/08/2023, 13,907 determinations of total serum IgE were performed in the analytical laboratory of the Provincial Hospital in Opole. It should be noted that this research was not a routine one. In the retrospective study presented here, it was decided to compare clinically two groups - those with the ultra-extremely low IgE level (<0.1 U/l), i.e. virtually undetectable, and the extremely high IgE (defined as >10,000 U/l). Results: Women predominated in the group with ultra-extremely low IgE, men in the group with extremely high IgE. Clinically, respiratory conditions predominated in the first group, skin conditions in the second. Immunodeficiencies were more common in the first group, and likewise - to a lesser extent - oncological concerns. Conclusion: We propose that the IgE values <0.1 U/l should be defined as ultra-extremely low (or undetectable), the values between 0.1 and 0.5 as extremely low, and the values between 0.6 and 2.0 as low. Further observations should identify the clinical features and risk of significant health concerns in these individual patient groups.