A large part of human disease is chemical rather than bacterial in origin, and in diseases of chemical origin the trouble may be either excess or deficiency. Twelve case histories are here given to illustrate the fact that deficiency states resulting from poor diets are still important and sometimes puzzling. Mental symptoms disappeared when a man was finally treated with nicotinic acid for a long-standing dermatitis which was pellagra. A man of excessively frugal habits suffered from spells of increasing weakness; he was able to return to work after a series of hemorrhages led to the diagnosis of scurvy, which was remedied promptly by the oral administration of ascorbic acid. In many cases, however, the deficiency is more complex; the effect on the patient is a general biological blight, and a spectacular response follows the administration of complex foodstuffs such as nonfat dry milk solids. In some deficiencies anorexia and glossitis are prominent symptoms; breaking this vicious circle with folic acid in one case led to the appearance of a voracious appetite and the complete restoration of a patient who had been near death from severe macrocytic anemia. The body cells can recover to an amazing degree from these chemical upsets. The deficiency diseases bring crushing burdens and stark tragedies; their prevention is a significant part of actual medical practice.