1. Schizophrenia and bipolar disorder are neurodevelopmental disorders with significant genetic vulnerabilities. Several trophic genes and/or proteins have been implicated in the causation for both disorders.2. We hypothesized that these genes and/or proteins may impact neuronal growth in both disorders.3. Hippocampal tissue sections from CA1 area of schizophrenic, bipolar, depressed, and controls subjects, matched for age, sex, PMI, drug exposure, and brain pH were prepared for cell size determination using the Stanley Medical Research Foundation postmortem brain collection.4. Quantification of hippocampal CA1 pyramidal neuron size showed a significant 12% reduction in cell size (p < 0.05) in bipolar subjects vs. controls. There were nonsignificant trends for reduction in cell size in both schizophrenic and depressed subjects vs. controls.5. These results indicate for the first time that pyramidal cell atrophy is present in hippocampus of subjects with bipolar disorder.