interpreted with this caveat. Nevertheless, in other areas of Kenya where stable transmission is well established (51), notably coastal Kwale (ACR = 4,181/6,692 = 0.63 based on admissions data, 1984–1999) and lakeside Homa Bay (ACR = 18,686/35,703 = 0.52 based on admissions data,1982–1999), many more children than adults are admitted to hospital with a malaria diagnosis, resulting in ACRs similar to those described in the highlands (R. Snow, unpub. data). Conversely, in an arid area of northeastern Kenya (Wajir), where a major malaria epidemic occurred in 1998, more adults than children were admitted to the hospital (ACR = 2,704/1,369 = 1.96 based on admissions data, 1988–2000) (52). Despite poor malaria diagnosis in many routine clinical facilities, we believe that the ACR is one possible tool to rapidly assess the extent to which a community has sufficient parasite exposure to invoke some degree of clinical immunity early in childhood. This tool should be explored further within the context of malaria classification for epidemic-prone areas of Africa. In high-altitude zones of western Kenya, clinical malaria has an acutely seasonal distribution, is comparatively concen- trated in the pediatric population, and is a substantial public health problem every year. Occasional, but exceptional, tem- poral surges of disease occur in some years. We can assume that parasite transmission in this area of Kenya is stable and a degree of functional immunity is acquired during early child- hood. Low levels of parasite challenge have been found to be sufficient for early development of functional immunity (53). We argue that large parts of the western highlands, located at a similar altitude, have ecologies similar to many other areas with low, stable, but seasonal malaria in Kenya. Treating the highland districts as special cases; demanding intensive invest- ment in early detection, warning, and forecasting systems; and frequent complex-emergency responses by government or nongovernmental organizations (33) may not be the most appropriate and cost-effective use of limited resources. Invest- ment in sustainable approaches to vector control (spraying households with residual insecticide), promoting individual protection (insecticide-treated bed nets), and effective case management are perhaps more likely to achieve long-term reductions in disease. Acknowledgments The authors thank the staff of Tabaka Mission Hospital, St. Joseph’s Mission Hospital, and the Ministry of Health staff at Kisii District Hospital for assistance and dedication in identifying clinical records for this study. 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