BACKGROUND. Adequate representation of women in research has been deemed essential. METHODS. Cancer research published in 8 journals in 2006 was reviewed. The percentage of women among study participants was compared with the proportion expected from population-based estimates of sex-specific cancer incidence, using binomial tests. Differences were assessed in sex distribution of participants by funding source, author sex, and focus of research with the Student t test, and in a linear regression model controlling for cancer type. RESULTS. A total of 1534 cancer research articles were identified, of which 661 (representing 1,096,098 participants) were prospective clinical studies and were analyzed further. For all 7 non-sex???specific cancer types assessed, the majority of studies analyzed included a lower proportion of women than the proportion of women among patients having cancer of that type in the general population. Among studies focusing on cancer treatment, women constituted a significantly lower overall proportion of the participants in the analyzed studies than expected for 6 of 7 non-sex???specific cancer types ( P