Previous studies of church dropouts and the reinvolvement of dropouts in church life are either qualitative in nature and/or deal with relatively limited age groups or historical periods. The 1978 Gallup survey of unchurched Americans provides a unique opportunity to explore the phenomena of religious disengagement and re-entry in a quantitative manner that includes the entire spectrum of the life cycle and fifty years of historical change. This paper takes that opportunity as its point of departure, focusing on life cycle and historical variation in dropout and re-entry rates, and the reasons given by dropouts for their disengagement. Defining a church dropout as someone who has stopped attending religious services for a period of two or more years, the study estimates that 46 percent of Americans drop out of active religious participation sometime during their lifetime, with the dropout rate being greatest among teenagers. The teenage peak in the dropout rate is found across all categories of our control variables. The swelling of the dropout rate during the teenage years appears to be due primarily to the lessening of parental influence characteristic of this stage in the life cycle and a generalized feeling that the church has little of interest or relevance to offer. Once past the teens, personal contextual reasons for disengagement (e.g., moving to a new community, change in work schedule, poor health) predominate, especially for those over fifty-four. The study finds little historical variation in the dropout rate from the 1930s through the 1950s. In the 1960s, however, there was a significant increase in the dropout rate, with only a slight abatement of this peak rate in the 1970s. The study strongly suggests that church disengagement is a temporary, rather than permanent, stage in one's life. Up to 80 percent of religious dropouts, depending upon age at disengagement, re-enter active church involvement. The re-entry rate is shown to be greatest among those 25 to 34 years old. The net gain-loss rate for the population as a whole is positive only during the 1970s.