We had observed that several patients with acute promyelocytic leukemia (APL) were markedly obese. Therefore we determined the relationship between obesity and a diagnosis of APL among patients with acute myelocytic leukemia (AML). Between 1980 and 1995 we saw 1245 patients with newly-diagnosed AML of whom 120 had APL. Increasing body mass index (BMI) was strongly associated with a diagnosis of APL (P= 0.0003). Like Douer et al (Blood 1996; 8: 303–313) we found APL to be more frequent in Latinos and younger patients (P< 10− 4 for both). Logistic regression indicated that increasing BMI, decreasing age, and Latino origin were each independently associated with a diagnosis of APL (multivariate P values< 10− 4,< 10− 4, 0.0035, respectively). Since the mean BMI in the non-APL patients (25.1) resembled that of the general US population, it appears that APL patients are ‘heavy’and not that non-APL patients are ‘thin’. Five of the APL patients (4.2%) had a BMI> 50 (vs none of the other 1125 AML patients). Given the distribution of BMI in the general US population ages 17–74, the probability that five of 120 normal adults would have a BMI> 50 is virtually nil. Excluding the five very heavy APL patients does not alter the conclusion that increasing BMI predicted for APL in patients with AML. Although the mechanism is unclear there appears to be an association between increasing BMI and a diagnosis of APL among patients with AML. There may also be an association between APL and obesity.