['Prolonged disruption of sleep in animal studies is associated with decreased neurogenesis in the dentate gyrus. Our objective was to determine whether insomnia severity in a sample of posttraumatic stress disorder (PTSD) patients and control subjects was associated with decreased volume in the CA3/dentate hippocampal subfield.', 'Volumes of hippocampal subfields in 17 veteran men positive for PTSD (41 +/- 12 years) and 19 age-matched male veterans negative for PTSD were measured with 4-T magnetic resonance imaging. Subjective sleep quality was measured by the Insomnia Severity Index (ISI) and the Pittsburgh Sleep Quality Index.', 'Higher scores on the ISI, indicating worse insomnia, were associated with smaller volumes of the CA3/dentate subfields (r = -.48, p < .01) in the combined sample. Adding the ISI score as a predictor for CA3/dentate volume to a hierarchical linear regression model after first controlling for age and PTSD symptoms accounted for a 13% increase in incremental variance (t = -2.47, p = .02).'] Is insomnia severity associated with a decreased volume of the CA3/dentate gyrus hippocampal subfield?