To the Editor: Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immune deficiency caused by mutations in the WAS gene. The clinical triad of microthrombocytopenia, eczema, and immunodeficiency defines the classical WAS phenotype, whereas the so-called “attenuated form,” X-linked thrombocytopenia (XLT), manifests with thrombocytopenia alone with or without mild eczema. An increased risk of lymphoproliferative disease (LPD) in patients with WAS/XLT has been described as early as 19611 and confers a poor prognosis to patients. However, the precise incidence of LPD in patients with WAS remains unknown. Through a retrospective study within the national registry of CEREDIH, the French National Reference Center for primary immune deficiency (see the Methods section in this article's Online Repository at www.jacionline.org for more details), we identified 13 patients with WAS/XLT who had LPD before allogeneic stem cell transplantation (alloSCT) among the 189 patients with WAS and 46 patients with XLT treated between 1988 and October 2017. Considering that patients with WAS/XLT were at risk before undergoing alloSCT, the crude rate and incidence ratio of LPD in untransplanted patients were 8.4% and 511 per 1 × 105 patient-years (95% CI, 272–873 per 1 × 105 patient-years), respectively, which is greater than in the general population (7.5 per 1 × 105 patient-years [95%CI 7.1–7.8]; P < .001). Crude rates (4.3% [95% CI, 0.5% to 14%] for the XLT cohort and 5.8% [95% CI, 2.9% to 10.2%] for the WAS cohort) and incidence ratios (334 [95% CI, 40–1200] and 531 per 1 × 105 patient-years [95% CI, 265–943], respectively) of LPD were similar in patients with XLT and those with WAS. The cumulative incidence of LPD was 1.9% at age 10 years and 9.9% at age 30 years, as shown by competing risk analysis. After age 40 years, the risk of LPD persists in untransplanted patients with WAS/XLT as 3 cases occurred at age 43.2, 50.3, and 50.6 years. The median age at diagnosis of WAS/XLT was similar in the entire cohort and in the subgroup of patients who developed LPD (1 year; interquartile range, 0–3 years).