Hyperdiploidy (chromosomal number 51‐65) is a common cytogenetic abnormality in pediatric patients with B‐lymphoblastic leukemia (B‐ALL) and belongs to the favorable cytogenetic subgroup. Hyperdiploidy in adult B‐ALL is much less common and its clinical significance has not been well studied. Among the 1205 patients with B‐ALL (1018 adults and 187 children) from our institution, 78 had a hyperdiploid karyotype, including 45 (4.4%) adults and 33 (17.6%) children (P < 0.0001). Among the patients with hyperdiploid B‐ALL, the adult group had a significantly inferior survival (similar to the patients with a normal karyotype) compared with the pediatric group (median survival: 42 months vs undefined, P = 0.0029). Hyperdiploidy in adults B‐ALL tended to more frequently harbor structural abnormalities (two or more) than children (53% vs 33%). Two or more structural abnormalities in a hyperdiploidy correlated with an adverse survival in adult patients (33 months vs undefined, P = 0.0008), similar to the survival of patients with a complex karyotype. We conclude that hyperdiploidy in adults with B‐ALL is less favorable and more commonly contains structural abnormalities comparing to pediatric patients. We suggest that hyperdiploidy with two or more structural abnormalities are best considered as a complex karyotype in adults with B‐ALL. Hyperdiploidy in adult B‐ALL is more frequently associated with structural abnormalities and a less favorable outcome comparing to it in children B‐ALL. Hyperdiploidy with two or more structural abnormalities is better to be classified as a complex karyotype in adult B‐ALL.