The aim of this study was to assess the proportion of subjects with transformed migraine (TM) who have 15 or more migraine days per month as a function of duration of chronic daily headache (CDH) in an adolescent sample. CDH is a syndrome characterized by 15 or more headache days per month. In specialty care, TM is the most common type of CDH. Most adults who meet criteria for TM do not meet the International Headache Society (IHS) criteria for chronic migraine (CM). TM criteria require 15 or more headache days per month (not necessarily migraine), with a current or past history of migraine. CM requires 15 or more migraine days per month. As TM develops, attack frequency increases and the number of migraine features diminishes. If this observation is correct, individuals who meet criteria for TM but not CM may be at a later stage in the evolution of the disease, compared with those who meet criteria for CM. We reviewed charts of 267 adolescents (13-17 years) seen in a headache centre, to identify 117 with TM. We divide subjects with TM into those with recent onset (</=1 year) vs. longer duration (>1 year) and examined the number of migraine days per month and demographic features. We modelled predictors of CM (>15 migraine days per month) using logistic regression. Of 117 adolescents with TM, 55 (47%) had recent-onset (<1 year) and 62 (53%) had long-duration TM. Those with recent-onset TM were much more likely also to meet criteria for CM (74.5% vs. 25.8%, P < 0.001). This was verified in the TM with medication overuse subgroup (recent onset 66.7%, vs. long duration 37%, P = 0.01) and in the TM without medication overuse subgroup (62.2% vs. 19.2%, P = 0.001). Modelling the dichotomous outcome of CM (>15 days of migraine/month) in logistic regression, CM was predicted by recent onset of CDH, recent onset of migraine (<36 months), and younger ages (</=15 years), but not gender or use of migraine preventive drugs or medication overuse. Among adolescents with TM, CM is more likely in individuals who are young, whose episodic headache began recently, and with CDH of recent onset. These findings suggest that early in the process of transformation, migraine is more frequent, and that as CDH evolves, fewer typical attacks of IHS migraine occur.