This study examined grammaticality and complexity of sentences in monolingual Spanish-speaking children with specific language impairment (SLI). A group of SLI children (n = 13), mean age 6, was compared to a control group (CCG) matched by age (n = 11), and a younger control group (LCG) with similar linguistic development (n = 13). Grammaticality and complexity of sentences were analysed including identification and counting of: a) simple and complex sentences, b) grammatical and ungrammatical sentences, and c) types of grammatical errors. SLI children were found to be more ungrammatical than CCG in both simple and complex sentences. Considering the number of errors in all sentences produced, SLI children commit more errors than both control groups. Complexity of sentences did not show statistical differences among groups. Future research should explore in further detail the types of errors made by monolingual Spanish-speaking SLI children.