Energy-dispersive X-ray microanalysis was used to determine calcium/phosphorous (Ca/P) ratios in undecalcified teeth, and the sulfur (S) content of dentin of decalcified teeth from normal patients and patients with familial hypophosphatemia, in an attempt to determine the effect of phosphorus deficiency. The results showed that normal enamel has a slightly elevated Ca/P ratio compared to pure apatite. Enamel from a tooth of an untreated patient with hypophosphatemia exhibited a significantly higher Ca/P ratio than the normal teeth whereas enamel from teeth of an intermittently treated patient exhibited Ca/P ratios similar to pure apatite. Surprisingly, globular dentin in the same teeth showed a Ca/P ratio similar to that of globular dentin of the untreated tooth. The decalcified dentin from teeth of three hypophosphatemic patients and eight normal patients showed a S peak which varied widely in concentration. No detectable differences could be found between normal and diseased teeth.