Hematological alterations can often be observed during rheumatic diseases. The effects can be clinically severe, ranging from anemia of different grades of severity, through increased risk of hemorrhage due to thrombocytopenia up to severe infections as a result of high-grade leukocytopenia. The clinical sequelae for patients are predominantly determined by the extent of cytopenia. The underlying disease itself can initially be considered as the cause. Examples are anemia as a result of chronic inflammation, antibody-mediated thrombocytopenia as in systemic lupus erythematosus (SLE) or granulocytopenia within the framework of Felty's syndrome. Immunosuppressive treatment also often leads to alterations in the blood constituents. Although some substances, such as cyclophosphamide can suppress all three cell types, there are also selective effects, such as isolated thrombocytopenia under treatment with tocilizumab and JAK inhibitors. The differential diagnostic clarification of cytopenia can be difficult and necessitates a systematic work-up of the course of the disease and the subsequent treatment. The reviews of anemia, leukocytopenia and thrombocytopenia presented here summarize the most important components of the differentiation of hematological alterations in patients with rheumatic diseases.