The haematopoietic growth factor, GM-CSF, has well-documented stimulatory effects on monocyte and macrophage functions. These effects include enhanced proliferation on their progenitor cells, increased endocytosis and metabolism of mature cells, increased function as antigen-presenting cells, and increased inhibition or killing of intracellular fungi, bacteria, protozoa and viruses. The major effect of GM-CSF on monocytes and macrophages is to enhance phagocytic and metabolic functions, including increased synthesis of molecules toxic to microbes, and to release other proinflammatory cytokines. This results in inhibition and/or killing of Candida albicans, Aspergillus, Cryptococcus, Pneumocystis, Leishmania, Mycobacteria, as well as other intracellular pathogens. GM-CSF also enhances the intracellular effectiveness of antiviral and antibacterial drugs. Viral replication may be increased in activated cells, therefore, when GM-CSF is used, a combination with appropriate antiviral drugs is recommended. Several reports in patients of successful management of microbial diseases which depend on macrophage function are now reviewed. These reports support the clinical value of GM-CSF in the management of patients with cancer and chemotherapy related monocyte/macrophage dysfunction and presumed or documented microbial disease.