The direct and indirect radiative forcing due to sulfate aerosols is calculated in a version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3). This model includes a sulfur chemistry model and predicts the mass of sulfate. New optical properties are presented that account for the hygroscopic growth effects on both extinction optical depth and asymmetry parameter. These new properties enhance the sulfate direct forcing for relative humidities above 90% compared to previous results. The global annual mean forcing is -0.56 W m-2. The forcing due to the indirect cloud albedo effect is studied using four different methods to relate cloud drop number concentration to sulfate mass. One method assumes the presence of background aerosols that can also act as a source of cloud condensation nuclei. This effect reduces the magnitude of the indirect effect by 40% to -0.4 W m-2. This sensitivity study indicates the importance of the presence of other aerosols that can nucleate cloud drops. The seasonal cycle of the indirect effect is different from that of the direct effect, as the maximum of the indirect effect occurs in the Northern Hemisphere springtime, while that of the direct effect is largest in the Northern Hemisphere summer. The four different methods of accounting for the indirect effect result in a large uncertainty in the global annual mean net forcing due to sulfates and greenhouse gases, 0.05 to 1.42 W m-2. It is argued that a less empirical and more physically based approach is required to account for the indirect effect in climate models. ¿ 2000 American Geophysical Union