The effects of aerosols on the optical properties of water clouds (so-called indirect aerosol effects) are a major current source of uncertainty in the radiative forcing of climate change. Uncertainty in the indirect radiative forcing by anthropogenic sulfate aerosols is analyzed for five different model structures with 20 input uncertain parameters using the probabilistic collocation method. The structural uncertainty, measuring the range of the mean responses in different models, is -1.2 to -1.7 W m-2; and the parametric uncertainty, induced by the uncertainties in the input model parameters, is -0.1 to -5.2 W m-2 with 95% confidence. This implies that refining the input parameters may be more important than improving models in order to minimize the uncertainty in the indirect radiative forcing by anthropogenic sulfate aerosols using these five models. The rankings of sensitivity to and variance contribution by the 20 uncertain parameters shows very different patterns. The variance analysis indicates that the parametric uncertainty comes mainly from aerosol size parameters and atmospheric transmittance, indicating the importance of developing size-resolved global aerosol models. Although the conclusions reached here are subject to the choice of model structures and uncertain parameter distributions, this study introduces a useful framework for computing uncertainty in a critical climate issue. ¿ 1998 American Geophysical Union