IGF-I and insulin, acting through both IGF-I and insulin receptors, have been studied widely to evaluate their oncogenic and teratogenic properties. These two properties need to be studied for each new insulin analogue, in addition to measurements of their metabolic and pharmacodynamic features. This editorial critiques a study in this issue of the journal of several insulin analogues in their action in vitro on several cancer-related cell lines. The conclusions and limitations of these studies are highlighted, especially as they influence guidelines for using these analogues patients. Copyright © 2009 John Wiley & Sons, Ltd.