Although it is well-established that children with Specific Language Impairment characteristically optionally inflect forms that require tense and agreement marking, their abilities with regards to derivational suffixation are less well understood. In this paper we provide evidence from children with Grammatical-Specific Language Impairment (G-SLI) that derivational suffixes, unlike tense and agreement suffixes, are not omitted in elicitation tasks. We investigate two types of derivation--comparative/superlative formation and adjective-from-noun formation--and reveal that G-SLI children supply these suffixes at high rates, equivalent to their language matched peers. Moreover, increasing the phonological or morphological complexity of the stimulus does not trigger suffix omission, although it results in non-target forms that are not characteristic of typically developing children. We discuss what these results reveal about the nature of the deficit in G-SLI within the context of three hypotheses of SLI: the Extended Optional Infinitive, Implicit Rule and Computational Grammatical Complexity Hypotheses. (Contains 5 tables, 2 figures and 2 notes.)