Uni-pronuclear embryos (n = 42) were analysed by fluorescence in-situ hybridization (FISH) with two to four chromosome pair-specific probes. Half of these embryos resulted from conventional insemination and half from intracytoplasmic sperm injection (ICSI). The majority of uni-pronuclear embryos from conventional insemination were normally diploid (61.9%) whereas only 9.5% of uni-pronuclear ICSI embryos (P < 0.001) were diploid. In addition, a significantly higher number of uni-pronuclear embryos from conventional insemination had a Y chromosome (10/21, 47.6%) when compared with ICSI embryos (2/21, 9.5%) (P = 0.015). It is concluded that the majority of uni-pronuclear embryos following regular in-vitro fertilization are fertilized, whereas those from ICSI are parthenogenetically activated. The latter embryos should not be considered for embryo replacement.