Acinetobacter spp. are Gram-negative non-fermentative bacteria which may be isolated as commensals from human skin, throat and intestine but are also increasingly responsible for hospital infections. Owing to frequent changes in their taxonomy, their pathogenic role in humans has not been clear but today acinetobacter is considered to be a significant nosocomial pathogen in outbreaks of hospital infections predominantly in intensive care units. Nosocomial infections due to acinetobacter include urinary tract infections, bacteraemia, wound and burn infections, but also they are frequently isolated from ventilator-associated nosocomial pneumonia. The frequency of hospital outbreaks of acinetobacter infections has required the development of reliable typing methods. As well as conventional 'phenotypic' methods (serology, biotyping, phage typing), 'genotypic' systems (ribotyping, plasmid profiles, pulsed-field gel electrophoresis) have been utilized for strain identification. These typing systems should allow a better understanding of the epidemiology of acinetobacter in the hospital environment, e.g. sources, modes of transmission, and result in more efficient preventive measures. Acinetobacter infections are difficult to treat owing to their frequent multiple resistance to the antibiotics currently available for the treatment of nosocomial infections; various mechanisms of resistance to beta-lactams and amino-glycosides have been identified in the genus. Combination therapy is usually recommended for treatment of acinetobacter nosocomial infections and active antibacterials include imipenem, ceftazidime, amikacin and the newer fluoroquinolones. Careful in vitro testing of the activity of combinations of these drugs is recommended prior to their use.