Wild yak (Bos mutus) are affected by a disorder known colloquially as “stiffness of extremities disease,” characterized by emaciation, lameness, stiffness in the gait, enlargement of the costochondral junctions, and abnormal curvature in the long bones. Results from preliminary epidemiologic and clinical observations suggested that this was a local, nutritional and metabolic disease associated with some mineral deficiency. Our objective was to determine the possible relationship between this disease and phosphorus (P) deficiency. We found that P concentrations in forage samples from affected areas were significantly lower than were those from unaffected areas, and the mean calcium:P ratio in the affected forage was 14:1. Phosphorus concentrations of blood, bone, teeth, and hair from affected yak were also significantly lower than were those from reference yak. Serum P levels of affected animals were much lower than were those of reference yak, whereas serum alkaline phosphatase levels were significantly higher than were those from reference yak. The P deficiency disease could be cured with supplement of disodium hydrogen phosphate (Na₂HPO₄). We conclude that the disease is mainly caused by P deficiency in forage.