Sporadic emergence of resistance during therapy with ciprofloxacin has been noted since its use in clinical trials began. It has occurred particularly, although not exclusively, with Pseudomonas aeruginosa and Staphylococcus aureus, both of which have MICs in the range 0.5-2.0 mg/l. Although not invariably associated with clinical failure of therapy, emergence of resistance has usually occurred in infections either where large numbers of organisms are present or in tissues where ciprofloxacin concentrations may not be optimal, or where both factors apply. Care in selection of patients, attention to optimal duration of therapy and adequate dosage may help to prevent emergence of resistance but combination therapy has not proven effective. Resistance may in some bacterial strains be permanent but in others frequently reverts to normal sensitivity. In some situations, spread to other patients is a significant problem and treatment in isolation (or at home) may be advisable. Emergence of resistance to ciprofloxacin in these species usually occurs in recognizable situations and, in such circumstances, the availability of alternative therapy and the quantitative risk of the emergence of resistance must be balanced against potential benefit. Ciprofloxacin should never be used for trivial infections caused by staphylococci or P. aeruginosa.