Introduction Obesity is a worldwide problem that is related to cardiac disease, thrombosis and cancer. However, little is known about the impact of obesity on the outcomes of adult acute lymphoblastic leukemia (ALL) patients. Methods We retrospectively evaluated a cohort of 154 newly diagnosed adult ALL patients between 1994 and 2011 at Mayo Clinic (Rochester). According to the World Health Organization (WHO) international BMI classification, patients were stratified as underweight, normal weight, overweight, and obese. For some analyses, patients were also stratified according to a two-sided non-obese or obese classification. Results The median follow-up time was 8.37 years. Obese patients were more likely to be women (p=0.024) and ≥60 years old (p=0.003). Five-year mortality rates were higher in obese patients than non-obese [HR 95% CI: 1.60 (1.03–2.50) p=0.035]. This was also the case in subgroup analysis among T-cell patients although the number of patients was small [HR 95% CI: 5.42 (1.84–15.98) p<0.001]. There was no difference in mortality among the B-cell patients. After adjusting for baseline variables, the difference in mortality remained in several models. There was no difference in EFS or cumulative incidence of relapse rates between obese and non-obese patients among the overall population. Discussion In conclusion, our study suggests that adult ALL patients with obesity have lower survival rates, especially in T-cell ALL.