A simple method for grading gastritis is to assess the severity of round cell infiltration and the loss of normal glands, and this may be applied to both antral and body changes. However, there is, as yet, no satisfactory classification of gastritis. In population samples, gastritis shows a linear increase in age-specific prevalence so that the annual increase in the body atrophic gastritis pool up to geriatric age is constant (1.5%). In the elderly, there appears to be a retardation of the process, particularly in the antral mucosa, where some healing trend is demonstrable. This dynamic behaviour is qualitatively similar in all population samples collected in Finland and Estonia. On the other hand, the dynamic behaviour of gastritis in different subpopulations differs markedly from that in the population at large. In pernicious anemia patients and their first-degree relatives, the progression of body atrophic gastritis in its final stages is about 20 times more rapid than in a general population, while, simultaneously, antral gastritis displays a distinct healing tendency. A behaviour opposite to that in pernicious anemia is seen in patients with active or healed duodenal ulcer disease and in duodenitis: antral gastritis behaves, on the whole, similarly to that in the general population, but in the body mucosa there occurs virtually no progression with age, and the mucosa generally remains normal or at the stage of superficial gastritis. However, after antrectomy body gastritis progresses rapidly in the remnant at first, but it slows down later and then closely resembles that in the general population.(ABSTRACT TRUNCATED AT 250 WORDS)