Humoral immunodeficiency with accompanying infections is an indication for human immunoglobulin replacement therapy. Whether treatment will be lifelong or necessary only temporarily depends on the nature of deficiency: primary (persistent) or secondary (persistent or transient). It is not always easy to distinguish between primary and secondary immunodeficiency, especially in adults. The article presents a case of a 39-year-old patient with anamnesis and medical tests results that suggested primary humoral immunodeficiency. The deficiency was diagnosed for the first time at the age of 38, when the patient was pregnant. The patient was qualified for immunoglobulin G replacement therapy. Clinical improvement was achieved. After the end of pregnancy, systematic improvement in immunological parameters was observed, suggesting the resolution of immunodeficiency. A decision was made to discontinue immunoglobulin replacement. Due to the ability to respond to vaccine, confirmed during diagnosis, preventive vaccines were recommended. There was no recurrence of serious infections. The clinical course finally enabled a diagnosis of secondary immunodeficiency. The presented case shows the importance of an active approach to the diagnostic and therapeutic process, constant assessment of clinical course, monitoring of IgG concentrations, and the awareness that in the situation when we do not have a genetic confirmation of the disease, the diagnosis may change.