Disruption of the insulin/IGF-I pathway increases life span in invertebrates. However, effects of decreased IGF-I signaling in mammalian models remain controversial. Using a rodent model with a specific and limited deficiency of GH and IGF-I, we report that GH and IGF-I deficiency throughout life [GH deficiency (GHD)] has no effect on life span compared with normal, heterozygous animals. However, treatment of GHD animals with GH from 4-14 wk of age [adult-onset (AO) GHD] increased median and maximal life span by 14% and 12%, respectively. Analysis of end-of-life pathology indicated that deficiency of these hormones decreased tumor incidence inGHD and AO-GHD animals (18 and 30%, respectively) compared with heterozygous animals and decreased the severity of, and eliminated deaths from, chronic nephropathy. Total disease burden was reduced by 24% in GHD and 16% in AO-GHD animals. Interestingly, the incidence of intracranial hemorrhage increased by 154 and 198% in GHD and AO-GHD animals, respectively, compared with heterozygous animals. Deaths from intracranial hemorrhage in AO-GHD animals were delayed by 14 wk accounting for the increased life span compared with GHD animals. The presence of GH and IGF-I was necessary to maximize reproductive fitness and growth of offspring early in life and to maintain cognitive function and prevent cartilage degeneration later in life. The diverse effects of GH and IGF-I are consistent with a model of antagonistic pleiotropy and suggest that, in response to a deficiency of these hormones, increased life span is derived at the risk of functional impairments and tissue degeneration. Copyright © 2005 by The Endocrine Society.