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The success of intracytoplasmic sperm injection (ICSI) is poor when only immotile spermatozoa can be retrieved. In a couple with complete male asthenozoospermia the possible use of the hypo-osmotic swelling test to select spermatozoa for microinjection was examined. Following incubation in hypo-osmotic medium (Hypo 10, IVF Science, Göteborg, Sweden), 26% of immotile spermatozoa showed signs of sperm swelling (HOS-positive). After injection of HOS-positive spermatozoa, 5 out of 12 oocytes fertilized (41%) and after transfer of three embryos a healthy singleton pregnancy was achieved. In a previous ICSI cycle of this couple without preselection of spermatozoa by the HOS test, only 1 out of 10 oocytes fertilized. It is concluded that selection of spermatozoa by hypo-osmotic swelling-test prior to sperm microinjection seems to be a valuable tool to increase the fertilization rate in cases with complete asthenozoospermia.