Despite the well-established use of functional appliances and extraoral headgears for the correction of Class II malocclusion in growing patients, there are only a few studies that have investigated this issue in an evidence-based manner. Twin studies can provide strong evidence in dentistry through the elimination of genetic variables, thus diminishing bias. Thus, aim of this twin study was to evaluate the short and long-term outcomes of the two approaches. Two monozygotic twin girls presenting Class II, division 1 malocclusion were blindly assigned to be treated, the first with a cervical headgear and the second with an activator during the 1st phase of treatment, which was followed by a 2nd phase with full fixed orthodontic appliances. Diagnostic records were collected at 4 different time points during treatment and retention period. For both girls the total treatment and retention periods were 2.75 and 2 years, respectively. The girls appeared with many differences after the end of the 1st phase of treatment, which were attributed to the different skeletal and dentoalveolar changes of each appliance used. However, after the end of total active treatment and after retention, they shared almost identical dental and skeletal characteristics. Although some distinct differences are present immediately following the use of functional appliances and cervical headgears for the correction of Class II malocclusion in growing patients, these are counterbalanced after comprehensive treatment and after retention. Thus, it seems that the initial different short-term results of the two treatment approaches disappear in the long-term.