This study evaluated somatic and dendritic growth of neurons in the frontoparietal cortex of Igf1–/– brains. Pyramidal neuron density was increased by ∼25% (P = .005) and soma size reduced by ∼10% (P < .001). Golgi staining revealed that cortical layer II–III neurons exhibited a significant reduction in dendritic length and complexity in Igf1 null mice. Dendritic spine density and presumably synaptic contacts were reduced by 16% (P = .002). Similar findings were obtained for cortical layer V and piriform cortex pyramids. Supporting a reduction in synapses, synaptotagmin levels were reduced by 30% (P < .02) in the Igf1 null brain. Investigation of factors critically involved in dendritic growth and synaptogenesis showed an ∼50% reduction in cortical CDC42 protein expression (P < .001) and an ∼10% reduction in brain cholesterol levels (P < .01) in Igf1 null mice. Evidence is presented that Igf1 deletion causes disruptions in lipid and microtubule metabolism, leading to impaired neuronal somatic and dendritic growth. Published 2003 Wiley-Liss, Inc.