This cephalometric study of individuals with Class II Division I malocclusions investigated changes occurring during and following treatment for 49 patients treated with either the Andresen activator or the Bionator (mean age initially 12·1 ± 1·1 years). The treated cases were compared with an untreated group of 24 patients (mean age initially 11·8 ± 0·5 years). Overjet reduction was found to occur mainly as a result of dento-alveolar change. Mean overjet relapse following treatment was small (0·8 mm), but individual variation was considerable (+5·2 to −3·7 mm). Changes in incisor inclination rather than antero-posterior skeletal change contributed to relapse. No evidence was found to suggest that vertical facial pattern or growth influenced relapse. Small, but significant correlations were found between the magnitude of overjet relapse and the reduction achieved during treatment (r = −0·42) and between the magnitude of upper incisor retroclination during treatment and proclination post-treatment (upper incisor to SN plane, r = −0·42). The only initial variable showing a significant correlation with overjet relapse was initial ovetjet and the value for this correlation was low (r = 0·31).