Current protocols for the culture of human zygotes to blastocysts use two-step sequential media systems. The efficacy of a one-step system involving potassium simplex optimized medium (KSOM(AA)) has been investigated. In study 1, development of zygotes from days 1 to 3 in KSOM(AA) was compared with that for medium P-1. In study 2, embryos were cultured from days 1 to 3 in P-1 followed by culture from days 3 to 5 either in KSOM(AA) or medium CCM. In study 3, the ability of KSOM(AA) to support development of embryos from days 1 to 5, without medium renewal, was compared with the sequential media system P-1-->CCM. The cell numbers and fragmentation scores of day 3 embryos were distributed similarly following culture in KSOM(AA) or P-1. Significantly more KSOM(AA) embryos exhibited cytoplasmic pitting. Blastocyst formation rates were not significantly different whether embryos were cultured in the P-1-->KSOM(AA) or the P-1-->CCM systems, or when cultured from days 1 to 5 in KSOM(AA) without medium renewal or in P-1-->CCM. Five babies have been born from nine blastocysts transferred after extended culture in KSOM(AA). A one-step protocol involving KSOM(AA) can be used successfully to culture human zygotes to the blastocyst stage.