Selective IgM deficiency (sIgMD) is classified under primary immunodeficiencies (PID). This study aimed to define the clinical and immunologic features of sIgMD. We assessed a retrospective medical record of patients who fulfilled the diagnostic criteria for sIgMD in a pediatric immunology department. There were 55 patients with sIgMD. Out of 55 patients, 13 (23.6%) patients, diagnosed with a well-defined PID disease, and nine, evaluated as transient hypogammaglobulinemia, were excluded in the follow-up. The ratio of the sIgMD was %0.12 in the outpatient clinic of pediatric immunology (33/27,000). Out of 33 patients, eight (24,2%) were asymptomatic during the follow-up period. Fifteen (45.4%) patients presented with upper/lower respiratory and skin infections. Six patients (18%) had chromosomal anomaly, or syndrome (trisomy 21, 22q11.2 deletion 1p deletion, CHARGE syndrome, and Cohen syndrome). Six (18%) had autoimmune/inflammatory diseases, such as Behcet's disease, immune thrombocytopenic purpura, Crohn's disease, Guillain-Barre syndrome, and diabetes mellitus. Five (15%) had allergic disorders. Three patients (9%) developed malignancy. The PID diagnoses were combined immunodeficiency, common variable immunodeficiency, chronic granulomatous disease, adenosine deaminase deficiency, and congenital neutropenia. Genetic disorders, autoimmune/inflammatory, and allergic diseases may accompany sIgMD. Approximately 25% of the patients were asymptomatic in our series. Patients had increased malignancy risk. We diagnosed about 25% of the patients having low IgM with a specific PID in the follow-up period. Thus, patients with sIgMD should be followed up regularly in immunology clinics.