Caspofungin (MK-0991; L-743,872) belongs to the echinocandin family, a new class of antifungal agents that act on the fungal cell wall by inhibiting glucan synthesis. Data in vitro, and experimental studies, have demonstrated that caspofungin has antifungal activity against yeasts of the genus Candida (including isolates resistant to azoles and amphotericin B), several species of filamentous fungi, including Aspergillus, and certain dimorphic fungi, such as Histoplasma, Blastomyces and Coccidioides. In vitro and in animals, caspofungin shows additive or synergic antifungal activity with amphotericin B and triazoles. It also possesses activity against Pneumocystis carinii. Clinical trials have shown caspofungin to be well tolerated and effective in invasive aspergillosis in patients refractory or intolerant to standard treatment (45% favourable responses), in oropharyngeal and oesophageal candidiasis (67-93% favourable responses with an efficacy similar to those of amphotericin B and fluconazole), and in invasive candidiasis with efficacy equivalent to that of amphotericin B, and better tolerability. The results of these first clinical trials were promising, and led to the approval of caspofungin for invasive aspergillosis after failure of, or intolerance to, standard therapy. Further studies are required to define the exact role of caspofungin in the antifungal armamentarium.