Introduction: Traditional dose-limiting toxicity (DLT) definition, which uses grade (G) 3-4 toxicity data from cycle 1 (C1) only, may not be appropriate for molecularly targeted agents (MTAs) of prolonged administration, for which late or lower grade toxicities also deserve attention. Patients and methods: In collaboration with pharmaceutical companies and academia, an European Organisation for Research and Treatment of Cancer (EORTC)-led initiative, Dose-Limiting Toxicity and Toxicity Assessment Recommendation Group for Early Trials of Targeted therapies (DLT-TARGETT); collected data from completed phase 1 trials evaluating MTAs as monotherapy. All toxicities at least possibly related to the study drugs that occurred during C1-6, their type, grade (CTCAEv3.0), and duration as well as patients' relative dose-intensity (RDI), were recorded. Results: The 54 eligible trials enrolled 2084 evaluable adult patients with solid tumours between 1999 and 2013, and evaluated small molecules (40), antibodies (seven), recombinant peptides (five) and antisense oligodeoxynucleotides (two). A maximum tolerated dose was set in 43 trials. Fifteen percent of the patients received <75% of the intended RDI in Cl, but only 9.1% of them presented protocol-defined DLTs. After Cl, 16-19% of patients received <75% of the intended RDI. A similar proportion of G >= 3 toxicities was recorded in Cl and after Cl (936 and 1087 toxicities, respectively), with the first G 3 toxicity occurring after Cl in 18.6% of patients. Conclusion: Although protocol-defined DLT period is traditionally limited to Cl, almost 20% of patients present significant reductions in RDI at any time in phase 1 trials of MTAs. Recommended phase 2 dose assessment should incorporate all available information from any cycle (notably lower grade toxicities leading to such RDI decrease), and be based on achieving >75% RDI. (C) 2014 Elsevier Ltd. All rights reserved.