More than 130 medicinal functions are thought to be produced by medicinal mushrooms (MMs) and fungi, including antitumor, immunomodulating, antioxidant, radical scavenging, cardiovascular, antihypercholesterolemic, antiviral, antibacterial, antiparasitic, antifungal, detoxification, hepatoprotective, antidiabetic, and other effects. Many, if not all, higher Basidiomycetes mushrooms contain biologically active compounds in fruit bodies, cultured mycelia, and cultured broth. Special attention has been paid to mushroom polysaccharides. Numerous bioactive polysaccharides or polysaccharide-protein complexes from MMs seem to enhance innate and cell-mediated immune responses, and they exhibit antitumor activities in animals and humans. While the mechanism of their antitumor actions is still not completely understood, stimulation and modulation of key host immune responses by these mushroom compounds seems to be central. Most important for modern medicine are polysaccharides and low-molecular weight secondary metabolites with antitumor and immunostimulating properties. More than 600 studies have been conducted worldwide, and numerous human clinical trials on MMs have been published. Several of the mushroom compounds have proceeded through phase I, II, and III clinical studies and are used extensively and successfully in Asia to treat various cancers and other diseases. The aim of this review is to provide an overview of and analyze the literature on clinical trials using MMs with human anticancer, oncoimmunological, and immunomodulatory activities. High-quality, long-term, randomized, double-blind, placebo-controlled clinical studies of MMs, including well-sized population studies are definitely needed in order to yield statistical power showing their efficacy and safety. Clinical trials must obtain sufficient data on the efficacy and safety of MM-derived drugs and preparations. Discussion of results based on clinical studies of the anticancer, oncoimmunological, and immunomodulating activity of MMs are highlighted. Epidemiological studies with MMs are also discussed.