This is one of a series of articles explicating a theory of family behavior under conditions of sustained uncertainty brought about by the diagnosis of a chronic, life-threatening illness of a child. The procedures of the grounded theory method were used to collect and analyze data. The analysis illuminated a perceptual–interpretive–behavioral process that made visible the emergence and intensification of the uncertain state during the prediagnostic period. This process, which may consist of up to three stages, most frequently began with the parents' initial awareness of the child's illness cues and terminated with the diagnostic announcement. The stages were categorized as Lay Explanatory (I), Legitimating (II), and Medical Diagnostic (III). The duration of stage III was found to be a significant factor in explaining the variation in parental response to the diagnosis. ©1995 John Wiley & Sons, Inc.