In 2005, the National Academies released the report Guidelines for Human Embryonic Stem Cell Research, which offered a common set of ethical standards for a field that, due to the absence of comprehensive federal funding, was lacking national standards for research. In order to keep the Guidelines up to date, given the rapid pace of scientific developments in the field of stem cell research, the Human Embryonic Stem Cell Research Advisory Committee was established in 2006 with support from The Ellison Medical Foundation, The Greenwall Foundation, and the Howard Hughes Medical Institute. This letter report is the committee's first set of amendments to the Guidelines and clarifies earlier recommendations and conclusions, including the criteria for determining which stem cell lines it is acceptable to use. Future deliberations of the committee will address items for which additional information gathering and more extensive debate and discussion will be necessary.