A late-onset syndrome, consisting of muscle weakness, muscle pain, and unaccustomed fatigue, has been reported with increasing frequency among former poliomyelitis patients. A population-based cohort of poliomyelitis patients from Allegheny County, Pennsylvania, was traced and surveyed to estimate the prevalence and incidence and to identify determinants of the post-polio syndrome. A questionnaire validated in clinical examinations of 40 cohort members was used in the survey. The prevalence of the post-polio syndrome was 28.5% of all paralytic cases (95% confidence interval 24.4-32.6). The risk of post-polio syndrome was significantly higher among patients who sustained substantial permanent impairment after polio and among females. The incidence did not vary with age at acute onset, acute severity, or level of physical activity after recovery. The strongest determinant of post-polio syndrome onset was the length of the interval following the acute illness, with incidence peaking at 30-34 years. Of all cases of post-polio syndrome, 79% reported no major change in impairment status since onset. This study demonstrates that poliomyelitis patients are not equally susceptible to post-polio syndrome within the interval of 30-40 years after the original illness. For syndrome cases, the onset was associated with new neuromuscular symptoms and functional changes but not with major new impairment.