Marine anemia, also known as plasmacytoid leukemia, is a recently described disease of farmed Pacific salmon in British Columbia. Most of what is known about the disease has been generated through laboratory studies or field investigations of severely affected farms. The goals of this study were to determine the range of the spatial and temporal distribution of naturally occurring marine anemia, identify potential risk factors, and provide an initial description of the impact of the disease on commercial salmon farms in British Columbia. Data were obtained from mail surveys, farm visits, and reviews of clinical and laboratory records. An attempt was made to evaluate negative, as well as mildly, moderately, and severely affected sites. The results showed marine anemia to be widely distributed throughout the major salmon farming regions in British Columbia. The disease was most commonly diagnosed in August and September, when water temperatures were at their seasonal peaks. A wide variety of lineage's and fish sources were associated with the disease. The average mortality rate attributed to marine anemia was 6% (range 2.5% to 11%). The peak occurrence of the disease was associated with a peak in the occurrence of other infectious and inflammatory diseases. The broad demographic distribution of marine anemia, coupled with its endemic nature, indicated that the disease is unlikely to be due to the recent introduction of a new pathogen and that causal factors are widespread in southern British Columbia. It is concluded that the significance of diagnosing marine anemia is not that it is predictive of an impending epidemic of mortality, but that it is an indicator of the general pattern of disease on a farm.