Spermine oxidase (SMO) was discovered much more recently than other enzymes involved in polyamine metabolism; this review summarizes 10 years of researches on this enzyme. Spermine oxidase (SMO) is a FAD-dependent enzyme that specifically oxidizes spermine (Spm) and plays a dominant role in the highly regulated mammalian polyamines catabolism. SMO participates in drug response, apoptosis, response to stressful stimuli and etiology of several pathological conditions, including cancer. SMO is a highly inducible enzyme, its deregulation can alter polyamine homeostasis, and dysregulation of polyamine catabolism is often associated with several disease states. The oxidative products of SMO activity are spermidine, and the reactive oxygen species H(2)O(2) and the aldehyde 3-aminopropanal each with the potential to produce cellular damages and pathologies. The SMO substrate Spm is a tetramine that plays mandatory roles in several cell functions, such as DNA synthesis, cellular proliferation, modulation of ion channels function, cellular signaling, nitric oxide synthesis and inhibition of immune responses. The goal of this review is to cover the main biochemical, cellular and physiological processes in which SMO is involved.