Recent research into fluoroquinolone antibacterials has led to the discovery of a number of compounds with greatly improved potency, spectrum, pharmacokinetics and clinical efficacy. In general their safety and tolerance mirrors these attributes. Like previous naphthyridines, cinnolines, and fluoroquinolones, these agents may cause gastrointestinal, central nervous system, and cutaneous reactions, but these are usually mild and self-limiting, affect 5-10% of patients only and rarely require withdrawal from therapy. Extremely rarely, more serious CNS effects including fits and psychosis may accompany high dose therapy. Potentially serious problems, predicted by animal testing and including erosive arthropathy in juvenile rats and dogs, cataract formation and renal damage secondary to crystalluria, have not been encountered in clinical use. All of the new agents give positive results in some tests for mutagenesis but there is no evidence for mutagenicity or carcinogenicity in man. However, the possibility of mutagenesis and joint damage restricts the use of fluoroquinolones in children to life-saving indications only. Interactions may occur with other drugs. Absorption of these agents is interfered with by co-administered antacids, although not by H2 antagonists. Oxo-metabolites of enoxacin and, to a lesser extent, pefloxacin and ciprofloxacin interfere with the hepatic elimination of theophylline and caffeine and may result in toxicity due to these agents if dosage is not modified. With the exception of these avoidable events, the fluoroquinolones have proved a particularly safe group of agents in clinical use.