On the basis of results from animal experiments and studies in human subjects, the amount of nitrosoproline (NPRO) excreted in 24-hr urine following ingestion of precursors (proline, nitrate) has been measured as an index of endogenous nitrosation. Several protocols of the NPRO test have been applied to human subjects, in order to study the kinetics and dietary modifiers of endogenous nitrosation, and in clinical and epidemiological studies. These studies have demonstrated that endogenous nitrosation in humans is highly complex and is influenced by factors such as gastric pH and amounts of precursors, catalysts and inhibitors. Thus, individual monitoring for nitrosation potential, rather than analyses of precursors in saliva, urine and gastric juice, is necessary in order to establish a causal relationship between endogenous nitrosation and human cancer. Results obtained after application of the NPRO test to subjects at high risk for cancers of the stomach, oesophagus, oral cavity and urinary bladder are summarized.