The hypoosmotic swelling (HOS) test is a relatively new assay used to evaluate the functional integrity of the sperm's plasma membrane. In fact, more studies have been published on the applicability of the HOS test than any other new sperm indicator. The assay is based on the fact that fluid transport occurs across an intact cell membrane under hypoosmotic conditions until equilibrium is reached. Due to the influx of fluid, the cell will expand and bulge, especially in the tail, and this change can be readily observed with a phase contrast microscope. Earlier studies have yielded some confusion regarding the interpretation of the data. This review is an attempt to clarify and update the usefulness of the HOS test as a tool to evaluate the sperm function.