The standard treatment in children with average-risk medulloblastoma (MB) is reduced-dose radiotherapy (RT) followed by chemotherapy. However, in adults, there is no agreement on the use of adjuvant chemotherapy. We performed a retrospective analysis of adult MB patients with average-risk disease, defined as no postsurgical residual (or ≤1.5 cm 2 ) and no metastatic disease (M0). Main inclusion criteria were: age >16 years, post-surgical treatment with craniospinal irradiation with or without adjuvant chemotherapy (cisplatin and etoposide ± cyclophosphamide). From 1988 to 2012 were accrued 43 average-risk MB patients treated with surgery and adjuvant RT. Fifteen (34.9 %) patients received also chemotherapy: 7 before RT, 5 after RT, and 3 before and after RT. Reasons to administer chemotherapy were presence of residual disease (even if ≤1.5 cm) and delay in RT. After a median follow up time of 10 years (range: 8–13), median survival was 18 years (95 % CI 9–28) in patients who receive RT alone, and was not reached in patients treated with RT plus chemotherapy. The survival rates at 5, 10 and 15 years were 100 %, 78.6 % (95 % CI 60.0–97.2 %) and 60.2 % (95 % CI 36.9–83.5 %), in patients treated with RT alone, and 100, 100 and 100 %, in patients treated with RT plus chemotherapy (p = 0.079). Our findings suggest a role for adjuvant chemotherapy in the treatment of average-risk MB adult patients. Further improvements might drive to add chemotherapy in average-risk setting with less favourable biological signatures (i.e., non-WNT group).