BACKGROUND: Underreporting of childhood sexual abuse is a major barrier to obtaining reliable prevalence estimates. We tested the sensitivity and specificity of the face-to-face-interview (FTFI) method by comparing the number of disclosures of forced sex against a more confidential mode of data collection, the sealed-envelope method (SEM). We also report on characteristics of individuals associated with non-disclosure in FTFIs. METHODS: Secondary analysis of data from a cross-sectional survey conducted in 2014, with n = 3843 children attending primary school in Luwero District, Uganda. Sensitivity and specificity were calculated, and mixed effects logistic regression models tested factors associated with disclosure in one or both modes. RESULTS: In the FTFI, 1.1% (n = 42) of children reported ever experiencing forced sex, compared to 7.0% (n = 268) in the SEM. The FTFI method demonstrated low sensitivity (13.1%, 95%CI 9.3-17.7%) and high specificity (99.8%, 95%CI 99.6-99.9%) in detecting cases of forced sex, when compared to the SEM. Boys were less likely than girls to disclose in the FTFI, however there was no difference in prevalence by sex using the SEM (aOR = 0.91, 95%CI 0.7-1.2; P = 0.532). Disclosing experience of other forms of sexual violence was associated with experience of forced sex for both modes of disclosure. CONCLUSIONS: The SEM method was superior to FTFIs in identifying cases of forced sex amongst primary school children, particularly for boys. Reporting of other forms of sexual violence in FTFIs may indicate experience of forced sex. Future survey research, and efforts to estimate prevalence of sexual violence, should make use of more confidential disclosure methods to detect childhood sexual abuse.