This study investigates how syntactic complexity affects speaking performance in first (L1) and second language (L2) in terms of speaking fluency. Participants (30 Dutch native speakers with an average to advanced level of English) performed two speaking experiments, one in Dutch (L1) and one in English (L2). Syntactic complexity was operationalized by eliciting active and passive sentences in an experimental setting. By comparing the effect of syntactic complexity on different measures of fluency, the results are telling of underlying cognitive processes in on-line speech production. We found that syntactic complexity indeed elicits hesitations, both in the L1 and in the L2. Because producing a rather simple utterance such as an active sentence may already lead to processing difficulty in the L2, the effect of syntactic complexity was found to be larger for L1 speech. Finally, articulation rate was not affected by syntactic complexity, neither in the L1 nor in the L2.