To investigate the patients with language disorder and to detect the activated areas in language processing, we measured the changes in the regional cerebral blood flow (CBF) during speech task (¿counting¿ task) using O-15 water PET activation technique in six normal subjects (age 58.3 +/- 8.1), mean +/- SD), ten fluent aphasics (age 60.3 +/- 12.5) and six nonfluent aphasics (age 50.5 +/- 8.3). In ¿counting¿ task, the subjects were instructed to count the number aloud from 1 to 10 and repeat the sequence over and over in the native language (Japanese) at the rate of one number per 2.5 sec. The data were analyzed with stereotactic intersubject averaging analysis for the normal subjects. Apart from it, the regions of interest (ROI) analysis (a circular ROI of 12 mm diameter) was performed in the language-related areas: posteroinferofrontal areas (PIF), posterosuperotemporal area (PST), and Rolandic areas (related to the mouth and lips) were significantly activated bilaterally in the ¿counting¿ task. PIF were activated with a dominance in the left side. In the resting state, rCBF in the left PIF and left PST were activated with a dominance in the left side. In the resting state, rCBF in the left PIF and left PST was reduced in both fluent and nonfluent aphasics. In nonfluent aphasics, the magnitude of activation in the right PIF by ¿counting¿ task was significantly greater than normal subjects and patients with fluent aphasia (ANOVA). It suggests the importance of the right PIF in the simple language processing in the nonfluent aphasia. In the sixteen aphasic patients, the increase in rCBF in the right PST during the ¿counting¿ task was negatively correlated with the score of comprehension (WAB) in the Spearman ranked correlation (p < 0.05). It suggests that, while the activity of the right PST is minimal for the ¿counting¿ in normal subjects, it plays an important and compensatory role in aphasic patients.