Stem-cell therapy is nothing new. The transplantation of bone marrow to treat blood cancers such as leukaemia and lymphoma has existed for decades: the therapy works because marrow contains blood-forming stem cells that can replace the cells being destroyed.

But the stem cells involved in such treatments simply continue to perform their natural function. What is new — and concerning — is a trend towards transplants in which the stem cells are expected to behave in different ways, even though there is little empirical evidence to suggest that they can do so. The perils of that approach were highlighted last week, when researchers reported the discovery of strange lumps of cells in the kidney of a woman who had undergone stem-cell treatment in Thailand (see Nature 465, 997; 2010). In another case last year, an Israeli boy developed tumours after being injected several times in a Moscow hospital with what was later determined to be a slurry of mixed fetal tissues.

There are at present estimated to be more than 200 clinics worldwide — including more than 100 in China alone — offering what are often unproven stem-cell treatments for scores of disorders including spinal-cord injury, amyotrophic lateral sclerosis and multiple sclerosis (see Nature 459, 146–147; 2009). The potential profits are huge: there is an abundance of patients desperate for miracle cures, and one stem-cell treatment can bring in tens of thousands of US dollars.

Practitioners at these clinics claim that their treatments are safe and effective. But they typically base those claims on little more than patient testimonials and media accounts, and they lack independent oversight. Few offer evidence from controlled clinical studies or from rigorous follow-up of their own patients.

Government regulation of stem-cell clinics has so far been inconsistent — and is difficult in any case, owing to their cross-border appeal. But two non-governmental organizations have now started offering information to help patients navigate the flood of nonsense and half-truths. In April, the International Cellular Medicine Society (ICMS), a group of 224 doctors and researchers based in Portland, Oregon, started an 'open treatment registry' intended to act as a clearing house for patient and clinician testimony (see Nature Med. 16, 495; 2010). The ICMS also accredits clinics that “provide complete disclosure of their collection, processing and re-implantation procedures, as well as all outcomes and complications data from patients they have treated”.

In principle, this represents a step in the right direction. But the ICMS needs to be rigorous about requiring clinical-trial data for the therapies offered by the clinics it certifies, working with regulatory agencies and demanding that patient follow-up be carried out by independent third parties. It should also insist that its member clinics put some of their profits towards testing their protocols rigorously — while thoroughly examining unexpected outcomes.

The primacy of such research is at the heart of another organization's mission. In June, the International Society for Stem Cell Research (ISSCR), a group of stem-cell biologists and clinicians based in Deerfield, Illinois, launched a website (see http://go.nature.com/zK3L4e) with its own evaluations of stem-cell therapies, including a list of questions for patients to ask their doctors. Visitors to the site are briefed on the role of publication, peer review and clinical trials in turning science into medicine. They can also submit prospective clinics and treatments for an ISSCR review, which will include a check of a clinic's ethical, regulatory and safety records.

To be considered credible, the ISSCR and ICMS must identify, and be transparent in dealing with, potential conflicts of interest. The ISSCR has made a good start, vowing to eject from the society members who are affiliated with clinics that offer unproven stem-cell treatments.

The medical promise of stem cells remains real, but largely unrealized for now. The excitement must not be left to dissolve into a muddle of disappointment, frustration and fear because of the practices of a few irresponsible profiteers.