Abstract From epidemiologic surveys, the prevalence rate for all seizures was about 360 cases per 100,000 population and the annual incidence (attack) rate about 30–36 per 100,000, which would suggest in general an average duration of some 10–12 yr for seizures. Age-specific prevalence rates were approximately equal throughout life except for a decline after age 60, of dubious significance. “Lifetime” prevalence rates for individuals who have or have had seizures were about 550 per 100,000. Average annual incidence rates for adults with seizures approximated some 15–18 cases per 100,000 adult population. Age-specific incidence rates for all seizures indicated a generally declining rate from a maximum in the first decade of life. This is probably the resultant of two separable classes of seizure disorder: primary grand mal and secondary seizures. The primary grand mal fell at a geometric rate from a peak in infancy to 0 at about age 69, with an overall annual incidence of about 18 per 100,000 population. Secondary seizures on the other hand occurred at about equal frequency throughout life save for a possible excess in the first decade, with an annual incidence rate of some 15 per 100,000. Distributions for age at onset in a series of hospitalized veterans were quite similar to those for adults in two epidemiologic surveys. In the veterans, primary grand mal and secondary seizures due to trauma began more often in the younger years, and fits with cerebrovascular disease in the elderly. Otherwise age was no indicator of etiology. Of the secondary seizures, half were grand mal alone, this being the most common type of seizure present for most etiologic categories; focal seizures were more common in brain tumors, and psychomotor fits in focal lesions of undetermined origin. When the kind of seizure present in the veteran series was considered, grand mal alone was the type found in 2 3 of the cases. Of the grand mal convulsions, half were classed as primary (no lesion), 1 4 were caused by trauma, and the remainder were the result of tumor, infection, vascular, toxic, degenerative, or unknown-origin lesions, in equally small numbers (3–7 per cent each). Focal seizures in the veterans were found to be most often the result of tumor (38 per cent) and trauma (32 per cent), with 18 per cent of undetermined origin. Psychomotor seizures were the result of trauma in 1 2 and of unknown cause in 1 3 of the cases; tumor was found in but 7 per cent of individuals with psychomotor seizures.