There is growing concern over the emission and accumulation of very long-lived fluorinated trace gases in the atmosphere, due to their large global warming potentials (GWPs). Unlike CFCs and other ozone-depleting, chlorinated and brominated chemicals, consumption of these fluorinated compounds is not controlled by the Montreal Protocol or any other international agreement. Of all the known and potential trace 'greenhouse' gases, the two with the highest GWPs are sulfur hexafluoride (SF) and fluoroform (CHF, HFC-23). Whereas several studies have reported the detection and accumulation of SF in the atmosphere, the presence of HFC-23 has remained unreported. We have found that present-day HFC-23 concentrations (c. 11 pptv in late 1995) exceed those of SF by a factor of three. Concentrations have steadily increased in the atmosphere since at least 1978, and are continuing to do so at a present rate of 5% per year. Furthermore, HFC-23 appears to be long-lived in the atmosphere, with a stratospheric lifetime of at least 1000 years, and a modelled tropospheric lifetime of 230 years. In terms of global warming, the cumulative emissions of HFC-23 up to, and including, 1995 are equivalent to 1.6 billion tonnes of CO.