The ADOS characterizes socio-communicative deficits in autism spectrum disorders (ASD). In this study the effect of module choice on ADOS classification was examined. For 74 participants (52 autism, 22 PDD-NOS), Module 1 and Module 2 were administered in a single session. Fifty-one participants maintained ADOS classification, with 17 more impaired on M2 and 6 more impaired on M1. For 64 participants (25 autism, 39 PDD-NOS), Module 2 and Module 3 were administered. Thirty-nine participants maintained classification, with 24 more impaired on M3 and 1 more impaired on M2. As expected, more impairment was indicated when a module with more language and task demands was administered. Clinical judgment of the most appropriate module for administration was found to be important.