Urinary tract infections (UTI) are very often encountered in patients with diabetes mellitus. They may present themselves as asymptomatic bacteriuria, but may also lead to more serious infections. In most cases the kidney is involved, although signs and symptoms of renal infection may not be present. Asymptomatic bacteriuria is more prevalent in women, but no men, with diabetes mellitus compared to controls. Studies yield conflicting results, probably due to selection bias, about associated risk factors. The pathogens cultured are those seen in all complicated urinary tract infections. The suggested mechanisms of an increased susceptibility to UTI are (a) decreased antibacterial activity due to the 'sweet urine', (b) defects in neutrophil function and (c) increased adherence to uroephithelial cells. The latter is the most likely. UTI in diabetics should be treated as complicated UTI with agents reaching high tissue levels for 10-14 days.