Elevated plasma cysteine to sulphate ratios were found in patients with Motor neurone disease (MND), Parkinson's disease (PD) and Alzheimer's disease (AD). Cysteine and sulphate were measured by colourimetric methods. Following recent discovery of a defect in sulphoxidation and sulphation of xenobiotics in these diseases, this finding confirms that endogenous sulphur metabolism is disturbed. The mean cysteine:sulphate ratios (× 10³) in fasting early morning plasma were 506, 521 and 477 for MND, PD and AD whereas it was 96 for normal controls (P < 0.001). This excess of cysteine thiol groups may interfere with neural protein function. The deficiency of sulphate ions may lead to reduced xenobiotic detoxification.