The hypo-osmotic swelling test was introduced as one of the sperm function tests more than 20 years ago to detect the functional integrity of the human sperm tail membrane. The aim of this study was to determine the effect of the time course on the interpretation of hypo-osmotic swelling test results. Semen samples from 30 infertile men were used in this study together with semen samples from 30 fertile control subjects as the control group. Semen samples were collected in the laboratory and those samples that contained coiled tailed spermatozoa were rejected. Semen was subjected to the HOS test after being subjected to normal semen analysis following WHO (1992) criteria. Samples were immediately evaluated for the HOS swelling or formalin fixed for later evaluation. The samples were evaluated every 5 minutes for 60 minutes. In this study, we used the type "b" and type "g" swelling in interpretation of our results. From the results, we obtained it was clear that sperm ta9l swelling and curling started to appear after 5 minutes from the beginning of the test and rises to reach its peak value at about 30 to 35 minutes then steadily declines till the 60 minutes. Also we concluded that the response of the sperms to the test was better in the semen of the control group than that of the semen of the infertile patients. Lastly, we concluded that the timed hypo-osmotic swelling test should be considered as one of the important and reliable sperm function tests that evaluate the functional integrity of the sperm and its ability to fertilize the ova.