The pro-inflammatory cytokine interleukin-1 beta has been implicated in the pathogenesis of major depressive disorder and in cognitive function decline in the elderly. This study tests the hypothesis that a biallelic functional polymorphism in the promoter region of the interleukin-1 beta gene (IL1B -511C/T) affects vulnerability to geriatric depression and its manifestations, including age of onset, depression severity, and cognitive function. We genotyped the IL1B -511C/T polymorphism in 125 elderly inpatients diagnosed with major depression and 282 normal elderly controls. The depressed patients were evaluated at baseline after admission using the Hamilton Rating Scale for Depression (HAM-D) for depression severity and the Mini-Mental Status Examination (MMSE) for cognitive function; depression age of onset was evaluated by interview and medical records. We found no association between IL1B -511C/T genotypes and geriatric depression susceptibility (P = 0.213), depression severity (HAM-D scores; P = 0.766) or cognitive function (MMSE scores; P = 0.827); however, compared with depressed subjects carrying the -511C allele, depressed subjects who were -511T homozygotes showed a significantly later depression age of onset of 7 years (P = 0.021). Our findings suggest that the IL1B -511C/T polymorphism may be related to age at manifestation among individuals vulnerable to depression, but they do not affect the basic vulnerability to or severity of depression in elderly Chinese adults. Further study is warranted to confirm this finding and to assess its generalization to other ethnic groups.