A laboratory investigation of the attenuation of mechanically generated waves by an opposing wind has been completed. Wave attenuation was quantified by measurements of the decline in surface variance. These measurements show higher effective levels of monochromatic wave attenuation than predicted by air-side measurements: approximately an order of magnitude higher than measurements by Young & Sobey (1985) and, a factor of 3 higher than those of Donelan (1999) for waves in a JONSWAP spectrum. Furthermore, they show that theoretical estimates currently underestimate the attenuation rates by a factor of at least 3. This study has shown that the magnitude of wave attenuation rates due to opposing winds is approximately 2.5 times greater than the magnitude of wave growth rates for comparable wind forcing. At high wave steepnesses, detailed analysis suggests that air-side processes alone are not sufficient to induce the observed levels of attenuation. Rather, it appears that energy fluxes from the wave field due to the interaction between the wave-induced currents and other subsurface motions play a significant role once the mean wave steepness exceeds a critical value. A systematic relationship between the energy flux from the wave field and mean wave steepness was observed. The combination of opposing wind and wind-induced water-side motions is far more effective in attenuating waves than has previously been envisaged.