Background: Adequate gender representation in clinical trials of new drugs is critical in order to accurately detect possible differences in response and toxicity (Özdemir et al, JCO 2018). The under-representation of women in oncology clinical trials has been previously described, however data on registrational trials, which are the basis for drug approval and inform the prescribing information, is lacking. We conducted an analysis of the trials supporting Food and Drug Administration approval of oncology drugs over a 5-year period to evaluate the representation of women vs. men. Methods: Prescribing information for novel new drugs approved from 2014-2018 was reviewed for the proportions of men and women in the evaluable population of the supporting clinical trials. Sex-specific cancers were excluded. Prevalence estimates for the indications were obtained from the Surveillance, Epidemiology and End Results database and the published literature. A participation to prevalence ratio (PPR) was calculated for each trial by dividing the percentage of women in the trial by the percentage of women in the disease population. A PPR value closer to unity represents even gender distribution and the range 0.8-1.2 is considered to reflect an acceptable representation of women. Data are presented using descriptive statistics. Results: A total of 46 oncology drugs were approved based on 56 trials enrolling 13,862 patients (7941 [57%] men; 5,921 [43%] women). Of the 56 trials, 38 (68%) had a PPR within the 0.8-1.2 range, 15 (27%) fell between 0.4-0.7, and 3 (5%) had a PPR of 1.3. The proportion of trials with unbalanced gender representation was comparable for hematological malignancy and solid tumor indications and did not improve over time. Fewer unbalanced trials were Phase III or employed a randomized design. Nine of the 18 (50%) unbalanced trials enrolled <100 subjects, compared to 3 of the 38 (8%) balanced trials. Conclusions: A third of registrational trials for oncology drugs lacked balanced gender distribution. Of the trials lacking balance, the vast majority (80%) had under-representation of women. Phase I-II trials and smaller trials had greater gender disparity, a concerning finding in a precision medicine environment where an increasing number of registration trials have double digit accrual. Further research is needed to understand the implications of unbalanced gender accrual in registrational trials, and to develop strategies for preventing disparities.