Arsenic can cause cancerous and non-cancerous human diseases. Inorganic arsenic from drinking water is the most common source of human exposure. Pentavalent arsenate can be reduced to trivalent arsenite in the blood, which is taken up mainly in the liver and metabolized by a sequence of reduction and oxidative methylation. A proportion of the inorganic arsenicals together with methylated metabolites are excreted in urine. Analyses of the urinary arsenic profile can give a hint to the methylation capacity of exposed individuals. All studies evaluating the association between urinary arsenic profiles and human diseases nowadays measure mainly the inorganic arsenate and arsenite and the two organic forms of methylated metabolites: the pentavalent form of monomethylarsonic acid (MMAV) and dimethylarsinic acid (DMAV). A review of the current literature suggests that reduced methylation capacity with increased MMAV percentage, decreased DMAV percentage, or decreased DMAV/MMAV is associated with skin lesions, skin cancer, bladder cancer, peripheral vascular disease, muscle cramps and structural chromosome aberrations in peripheral lymphocytes obtained from exposed subjects. The detection of the recently identified more toxic trivalent forms of methylated metabolites in urine awaits further confirmation.