Intractable epilepsy is related to various transient and chronic brain electric and neurochemical disturbances. There is increasing evidence that long-lasting chronic epilepsy may induce secondary neuronal loss and metabolic dysfunctions. Still a matter of controversy is, however, whether cognitive abilities of patients deteriorate with increasing duration of intractable epilepsy. We present results from two independent cross-sectional studies dealing with measures of global cognitive performance in two different ways. The first study investigated in 78 patients with refractory temporal lobe epilepsy (TLE) the influence of duration of epilepsy on the difference between an estimated measure of former or pre-morbid intelligence and the current performance in an intelligence test. The second study aimed at duration of epilepsy-dependent effects on current IQ measures of 209 patients with refractory TLE. Both studies showed that the duration of epilepsy contributes to the explanation of interindividual variability in IQ measures of adult TLE patients to a higher degree than age and age of epilepsy onset. Similar to several studies on hippocampal neuronal density, hippocampal volume, and glucose metabolism, the presented cross-sectional data demonstrate that a duration of chronic epilepsy exceeding two decades is associated with worse cognitive abilities. Consequently, refractory TLE seems to induce a very slow but ongoing cognitive deterioration. It is assumed that epilepsy-related noxious events and agents exhaust the compensatory capacity of brain functions. A high cognitive reserve capacity, however, might delay the onset of deterioration.