Ca/P atomic ratios in spinal cords of seven amyotrophic lateral sclerosis (ALS) cases were determined by neutron activation and X-ray fluorescence analyses, and were compared with those of seven control cases. In order to obtain calibration curves, the standard samples with 0.01-0.1 of Ca/P atomic ratio were prepared by mixing diammonium hydrogenphosphate with calcium carbonate. In neutron activation analysis, gamma-ray spectra of49Ca and beta-ray intensities of32P produced from (n, γ) reactions of the spinal cords and standard samples were measured by Ge (Li) and GM detectors, respectively. In X-ray fluorescence analysis, the samples were excited with 1.6 MeV alpha particle beam accelerated by a Van de Graaf accelerator, manganese K X-rays from55Fe radioactive source and rhodium Ka X-rays from X-ray tube, respectively. The characteristic X-ray spectra were measured by Si (Li) or proportional detector. From the peak areas on the spectra and beta-ray intensities, the Ca/P atomic ratios in spinal cords were determined. Based on these results, the Ca/P atomic ratio of precipitate including calcium and phosphorus in spinal cord of ALS case was calculated. As a result, the Ca/P atomic ratios in spinal cords of ALS and control cases were 0.018-0.021 and 0.030-0.049, respectively. The calculated Ca/ P atomic ratio in the precipitate of ALS case was estimated to be 1.6.