Platanthera hookeri (Torr.) Lindl. is a little-studied and increasingly rare species of the Great Lakes–St. Lawrence and Acadian Forest Regions. Long-term studies are essential to provide an understanding of its population and flowering dynamics. Three representative populations, with a total of 54 plants, were monitored in Gatineau Park for up to 25 years (1981–2005). Six life stages were identified: seed/protocorm, juvenile, immature, flowering, vegetative and terminal. This paper deals with the juvenile stage, represented by a total of 16 transient plants, and 38 mature plants in the latter three stages. In the two larger populations, mature plant counts remained constant and most mature plants flowered almost annually for a period of years. Subsequently, the counts declined steadily and most plants flowered every other year. The third population experienced a continuous slow decline. The percentage of plants flowering in the longest-studied population was correlated positively with the total rainfall, and negatively with the mean temperature, of the June–July period of the preceding year. These three populations showed differences in behavior that probably were based on differences in age and habitat. Possible causes of mortality include forest succession, herbivory by White-tailed Deer (Odocoileus virginianus) and acid rain.