Parkinson's disease is associated with the loss of dopamine neurons in the caudate and putamen nuclei of the brain. Several groups have tried to slow down the neuron loss by transplanting embryonic precursors of dopaminergic cells and obtained some promising results. In the 8 March issue of New England Journal of Medicine a team from University of Colorado School of Medicine, Denver, published the first double-blind placebo controlled study of the transplant therapy (N Engl J Med 2001, 344:710-719). Unfortunately, the results are negative.

Freed et al randomly assigned 40 patients to receive a transplant of nerve cells or undergo 'sham' surgery. In the transplant recipients, cultured mesencephalic tissue from four embryos was implanted into the putamen bilaterally. In the patients who underwent sham surgery, holes were drilled in the skull but the dura was not penetrated.

After one year, patients younger than 60 years old in...