['Computed tomography (CT) is an imaging modality involving ionizing radiation. The presence of γ-H2AX foci after low to moderate ionizing radiation exposure has been demonstrated; however it is unknown whether very low ionizing radiation exposure doses from CT exams can induce γ-H2AX formation in vivo in young children.', 'To test whether very low ionizing radiation doses from CT exams can induce lymphocytic γ-H2AX foci (phosphorylated histones used as a marker of DNA damage) formation in vivo in young children.', 'Parents of participating children signed a consent form. Blood samples from three children (ages 3-21\xa0months) undergoing CT exams involving very low blood ionizing radiation exposure doses (blood doses of 0.22-1.22\xa0mGy) were collected immediately before and 1\xa0h post CT exams. Isolated lymphocytes were quantified for γ-H2AX foci by a technician blinded to the radiation status and dose of the patients. Paired t-tests and regression analyses were performed with significance levels set at P\u2009<\u20090.05.', 'We observed a dose-dependent increase in γ-H2AX foci post-CT exams (P\u2009=\u20090.046) among the three children. Ionizing radiation exposure doses led to a linear increase of foci per cell in post-CT samples (102% between lowest and highest dose).'] Are γ-H2AX foci increased in lymphocytes in vivo in young children 1 h after very low-dose X-irradiation : a pilot study?