Several trends in clinical medicine have converged recently and placed intestinal protozoan infections in a position of increasing importance to health professionals. These trends include the pandemic of human immunodeficiency virus (HIV) infections that cause the acquired immunodeficiency syndrome (AIDS) and result in associated opportunistic infections. The increasing use of powerful chemotherapeutic and immunosuppressive agents to prevent rejection of transplanted tissues in human allograft recipients has predisposed these patients to intestinal parasitic infections, which often become chronic and debilitating. Large numbers of people engage in business, philanthropic work, and vacation travel on a worldwide basis. The number of susceptible, potential human hosts for parasitic infections will continue to increase in the coming years. We reviewed 4 protozoan infections that have recently attracted the interest of clinicians, either because they are newly discovered or because they are increasingly prevalent. These infections include cryptosporidiosis and recently described infections due to Cyclospora species. The AIDS pandemic has also been associated with both the discovery and the rapid emergence of human microsporidiosis. Isospora belli has received renewed attention because of chronic infections now observed in HIV-infected hosts.