During the eight year period 1969-76, 214 patients with primary acute pancreatitis were admitted to Nottingham hospitals from the Nottingham defined population area (the City of Nottingham and the four adjacent urban districts, which had a population of 469,720 in 1971). There was considerable variation in both the incidence and distribution of the disease within the study area and the crude average incidence rates ranged between 31.8 and 388.7 per million. Six of the eight wards with rates in excess of 200 per million were contiguous with similar wards; together these formed a U-shaped area extending from the city centre to the eastern boundary of the study area. These findings could not be accounted for by the age structure of the population or its social class structure. Although the Nottingham defined population area is relatively small (147 km2 or 57 sq. miles), it is subdivided into distinct and fixed domestic water supply areas. The distribution of patients among the six water supply areas showed that the Burton Joyce supply area, which is a particularly 'hard' water, contained significantly more patients than could have occurred by chance (P less than 0.002). This investigation of some social and geographical factors suggests that the chemical composition of the domestic water supply may affect the distribution of this disease within this particular urban area. This concentration of patients in the Burton Joyce water supply area suggests that some, as yet unidentified, property of the water supply may be an aetiological factor.