The GH/IGF1 axis may play an important role in cognitive function. This theory is supported by the finding that both GH and IGF1 receptors are located in several brain areas such as the hippocampus, a brain area that is known to play an essential role in cognitive processes, especially memory and learning. However, the exact mechanism by which the GH/IGF1 axis influences the cognitive functions is still unknown. Furthermore, little is known about the cognition in adults with both childhood-onset and adult-onset GH deficiency (CO-GHD and AO-GHD). Recent data indicate that cognitive function, particularly attention and memory, in adults with GHD might be impaired. To date, only a limited number of studies have been conducted to study the effects of GH replacement therapy on cognitive function in adults with GHD. In this paper, the results of studies on cognitive functioning in GHD patients, in particular the results of the studies performed in adults with CO-GHD, and the effects of GH replacement therapy in these patients, will be discussed. © 2008 European Society of Endocrinology.