Over the past 15 yr, a marked increase in crude mortality rates from Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS) has occurred in the U.S. This is often attributed to as yet undefined environmental factors. The deterministic risk of general mortality and mortality due to PD, ALS, ischemic heart disease (IHD), and stroke for the years 1963, 1977, and 1986 in the U.S., as defined by the method of longitudinal Gompertzian analysis, were calculated and compared. When the rise in PD and ALS mortality is viewed from the perspective of deterministic and competitive mortality dynamics, it becomes evident that the major force increasing mortality from these two neurologic diseases is the declining mortality from IHD and stroke. Consequently, there is no need to invoke intrinsic etiologic alterations in the environment to account for the observed increases in PD and ALS mortality. Recognition of the competitive nature of human mortality illustrates the inherent risk of making etiopathogenic conclusions based upon single disease mortality data.