We investigate the effect of clouds on the radiative forcing by tropospheric ozone increases between the pre-industrial and the present time, using the general circulation model ECHAM4. The results indicate that clouds attenuate the longwave forcing for clear sky conditions by about 20%, whereas it approximately doubles the shortwave forcing. The estimated total clear-sky and cloudy-sky forcings are 0.41 and 0.37 W m-2, respectively. Analysis of reported forcing estimates shows that the magnitude of the longwave cloud effect is 20--30% of the clear-sky longwave forcing. On the other hand, the shortwave cloud effect appears to depend largely on characteristics of the applied cloud distribution and the way clouds are treated in radiative transfer models, and varies between 30--250% of the clear-sky shortwave forcing. Hence, present estimates of the cloud effect range from zero to as high as 0.1 W m-2. ¿ 1999 American Geophysical Union