The use of positron-emission tomography (PET) has shown a variety of age-related alterations in neuronal activity in elderly patients. Near infrared spectroscopy (NIRS) has shown a multiplicity of evoked cerebral blood oxygenation (CBO) changes during cognitive performance. To evaluate the effects of aging on the CBO changes, we compared the CBO changes induced by cognitive tasks between young adults and older adults by using NIRS. Twenty-six participants performed 6 cognitive tasks to evaluate differences among young adults (mean age+/-SD, 28.8+/-4.4 years) and older adults (50.7+/-8.0 years). We monitored CBO changes in the left prefrontal cortex. We classified several common patterns of the NIRS parameter change and compared the frequency of each pattern in both groups. In both groups, the most common NIRS parameter change was an increase in oxyhemoglobin (Oxy-Hb) and total-hemoglobin (Total-Hb), with a decrease in deoxyhemoglobin (Deoxy-Hb). The frequency of this pattern in the older group, however, was considerably lower than in the young group (P<.01). In addition, decreases of Oxy-Hb and Total-Hb during the tasks were more common in the older groups, (P<.0001), suggesting a decrease of regional cerebral blood flow during the tasks. Age effects varied for specific tasks, with results being significant for verbal fluency and reading, (P<.05). During all tasks, older subjects were less likely than younger subjects to have the typical pattern of increases in Oxy-Hb and Total-Hb. In addition, compared with younger subjects, older subjects had a greater frequency of decreases in Oxy-Hb and Total-Hb. The overall results, as well as results for specific tasks, suggest that functional reorganization or alteration of CBO responses may occur during aging.