To investigate the efficacy of a low-protein diet suitable for long-term use, and to observe the effect in a double-blind manner of re-introducing large neutral amino acids or an inactive placebo amino acid, eight patients with 'on-off' phenomenon were given a low-protein diet suitable for long-term use, and then the diet was supplemented with large neutral amino acids (LNAA) or placebo in a double-blind crossover trial. Five patients had improved Parkinsonian disability on a low-protein diet compared to normal diet. On supplementing the diet with LNAA or placebo, three subjects' Parkinson's disease was significantly better on placebo and there was a trend towards improvement on placebo noticed subjectively by seven patients; however, there was no overall significant difference in disability measured by mobility diaries. Thus, we have been unable to show that the improvement on a low-protein diet is due to a reduction in LNAA, rather than a placebo effect, possibly due to the paucity of subjects; however, the patients' identification of the LNAA supplement suggests that it is a genuine effect.