This article describes the case of a 16-month-old Hispanic male toddler with cow's milk allergy living in northern California who was admitted to a children's hospital for weight loss and markedly elevated levels of serum alkaline phosphatase and parathyroid hormone. At a routine outpatient well-child visit, his mother expressed concern about a decrease in his appetite and activity level. A detailed diet history revealed that breast milk was his primary source of nutrition during his first year of life and he had not been given supplemental vitamins. With attempts to introduce cow's milk formula, he had developed a rash and swelling around the mouth. Shortly after his first birthday, his mother weaned him from breast milk and introduced unfortified rice milk as a palatable milk substitute. Upon admission he was pale and lethargic; his laboratory studies were remarkable for elevated serum alkaline phosphatase and parathyroid hormone and low levels of phosphorus, 25-hydroxy-vitamin D, and ferritin. Lower extremity radiographic studies were consistent with rickets. After 5 weeks of therapy with vitamin D(3) and iron, his serum 25-hydroxy-vitamin D level normalized. Within 12 weeks following therapy, the child demonstrated significant clinical improvement, with resolution of growth failure and bone reossification. His activity level had returned to normal. This case emphasizes the importance of adequate vitamin D intake for children with special attention to those who might have nutrition deficiencies attributable to milk allergy.