Information on the genetic patterns of asthma is fragmented, incomplete, and often contradictory. Genetic, molecular biological, and cell culture methods are now available to map, sequence, and define the function ofthe genes involved in the expression of asthma. Identification of these genes and their function will facilitate the development of new modes of treatment, and will lead to an understanding ofthe causal interaction between the genes and the environmental factors that are relevant in asthma. Investigators have encountered many problems in their study of the genetics of human diseases. One of the major 'lessons' that can be learned from past experience with the genetic analysis of allergy and other complex disorders is that false positive linkage results are possible, and replication of a linkage finding is extremely important. Given the major problems involved in study of the genetics of complex disorders such as asthma, a Collaborative Study on the Genetics of Asthma (CSGA) was initiated by the National Heart, Lung and Blood Institute (NHLBI) with input from the National Institute of Allergy and Infectious Diseases (NIAID). This cooperative study is to function as a unified, collaborative group and has set guidelines for the members involved. The CSGA provides the unique opportunity to replicate initial positive linkage results within this collaborative effort.