Preliminary data from two ongoing trials that aim to assess the safety of transplantation of human embryonic stem cells (hESCs) into human eyes to treat blindness are encouraging, according to Schwartz et al. One trial is for age-related macular degeneration, the other is for Stargardts macular dystrophy (SMD). The researchers prepared differentiated hESC-derived retinal pigmented epithelial (RPE) cells and tested them preclinically by transplantation into rat retinas to ensure the hESC-RPEs were pure and did not form teratomas. The stage of hESC-RPE differentiation mattered: cells with less melanin engrafted more efficiently than those with more melanin and were judged suitable for testing in humans. Two patients (one from each trial; each trial will eventually enroll 12 patients) had 50,000 hESC-RPEs transplanted into a part of one retina where degeneration was incomplete. Outcomes measured included demonstration of successful engraftment of hESC-RPEs, but only in the patient with SMD. At 4 months after engraftment, neither patient had any adverse outcomes, such as hyperproliferation, transplant rejection or teratoma. Both phase 1/2 trials will also assess clinical endpoints, such as visual acuity, for future efficacy trials. The trials will be fully enrolled in 2012, but pivotal phase 3 trials will then be needed to evaluate whether hESC-RPEs can improve clinical outcomes. (Lancet, doi:10.1016/S0140-6736(12)60028-2, published online 24 January 2012)