To assess the capability of perceiving forms in patients with visual field loss, a concept of functional visual field was introduced based on determinations of the time required for pattern recognition. Two series of stimulus patterns were made of Japanese syllabic hiragana characters drawn with black dots in the background of open circles of various sizes: the clear stimulus had only open circles in the background and the noisy stimulus had black dots scattered in the background. The stimuli were presented for various durations to 15 normal subjects and 25 patients with narrowed visual field; a correlation of the percentages of correct pattern recognition with the stimulus durations permitted calculations of the 50% recognition time. The recognition time was longer with the noisy than with the clear stimuli. The recognition time with a given stimulus size was longer in patients than in normal subjects. In 3 normal subjects the visual field was artificially narrowed and the recognition time was determined. The recognition time could be expressed by a power function of the ratio of the effective visual field diameter to the diameter of the stimulus pattern. On this basis the functional visual field size of a patient was defined as the size of the artificially narrowed visual field of the normal subject that required the same recognition time as that of the patient. The functional visual field of patients could be correlated with the area of the perimetric field with the V/4 target of Goldmann's projection perimeter. The concept of the functional visual field was found to be useful to express the patients' capability for pattern perception.