The present study compares the culture of human embryos using two different culture systems: a conventional culture incubator with humidified atmospheric air supplemented with 5% CO(2) and a closed culture incubator with a humidified 5% O(2) and 5% (5.2-5.5) CO(2) and 90% N(2) partial pressure. A total of 225 cryopreserved embryos were donated after informed consent and thawed in two batches of 100 and 125 embryos separated by 1 week. A total of 126 (56%) embryos survived the thawing procedure having at least one living blastomere. After thawing, the embryos were randomly allocated to culture in a 5% oxygen partial pressure (n = 65) or to culture in a 20% oxygen partial pressure (n = 61). The embryos were hereafter cultured for 4 days and the number of embryos developed to morula stage or blastocyst stage was examined. The results of the embryo culture in the two trials performed showed an increase from 43% to 68% (19 to 68%) morula formation in trial 1 (trial 2), i.e. on the whole the morula yield more than doubled in favor of culture in a 5% oxygen partial pressure compared with a 20% oxygen partial pressure.