Good evidence indicates that adolescents and young adults (AYAs) with cancer do badly compared with children with similar cancers. The reasons are poorly understood. Australian registry data on 14,824 cancers of adolescence and young adulthood seen between 1982 and 2002 were reviewed. A detailed substudy of clinical characteristics was analyzed from 179 AYAs with Hodgkin lymphoma (HL), Ewing sarcoma (ES) or osteosarcomas (OS) treated at a single institution. Despite significant improvements in survival for both groups over the period in question, for acute lymphoblastic leukaemia, rhabdomyosarcoma, ES, OS and HL, survival for AYAs was worse than for children. For ES, OS and HL, the survival gap occurred almost entirely in males (Hazard ratios compared with female AYAs of 1.8 [p < 0.01], 1.4 [p = 0.03] and 1.5 [p < 0.01] respectively). Survival outcomes from ES, OS and HL for female AYAs were not significantly different from children of either sex. For brain tumors and thyroid cancers, which are primarily treated surgically, there were no gender-related differences in outcomes. Although no differences in tumor stage or compliance were identified, male AYAs experienced less toxicity and lower response rates to chemotherapy (p = 0.008). Young males account almost entirely for excess mortality from chemosensitive cancers of adolescence and young adulthood compared to children, which may be due to relative underdosing with current chemotherapy dosing algorithms.