In a prospective multi-panel cohort study, we investigated how, from late middle age, individuals' health status improves or declines. In the Canadian National Population Health Survey, transition probabilities between different health states were estimated for 4330 people (58.8% women) aged 55+ at baseline over 2-year intervals from 1994 to 2000. Health status was defined by a deficit count, using 33 health-related variables combined in a frailty index. For each time interval, the chance of accumulating deficits increased linearly with the number of deficits. Older survivors (aged 70-85) showed a slightly lower chance of stability or improvement (52%; 95% confidence interval 50-54%) compared with those in late middle age (56%; 54-58%). Changes in health states can be described with high accuracy (R2=0.92) by a modified Poisson distribution, using four parameters: the background odds of accumulating additional deficits, the chance of incurring more or fewer deficits, given the existing number, and the corresponding probabilities of dying. An age-invariant limit to deficit accumulation was observed at 22 deficits. From late middle age, transitions in health states occur with a regularity that is easily modeled. Improvements in health can occur at any age. At all ages, there is a limit to deficit accumulation.