Long-term outcomes of constipation have not been fully evaluated. We investigated the incidences of Parkinson's disease, constipation-related surgery, and colorectal cancer (CRC) in patients with constipation and slow-transit constipation (STC), followed for up to 20 years.We collected data from 2165 patients (33.1% men; median patient age, 54 years; median symptom duration, 5.0 years) with a diagnosis of constipation (based on Rome II criteria) who underwent anorectal function test and a colonic transit time study, from 2000 through 2010, at a tertiary university hospital in Seoul, South Korea. The presence of STC was determined from colonic transit time. We used the Kaplan-Meier method to analyze and compare cumulative probabilities of a new diagnosis of Parkinson's disease or CRC according to the presence of STC. The patients were followed until the end of 2019.During a median follow up of 4.7 years (interquartile range, 0.7-8.3 years), 10 patients underwent constipation-related surgery. The cumulative probabilities of constipation-related surgery were 0.7% at 5 years and 0.8% at 10 years after diagnosis of constipation. Twenty-nine patients (1.3%) developed Parkinson's disease; the cumulative probabilities were 0.4% at 1 year, 1.0% at 5 years, and 2.6% at 10 years after diagnosis of constipation. At 10 years, 1.3% of patients with STC required constipation-related surgery and 3.5% of patients with STC developed Parkinson's disease; in contrast, none of the patients without STC required constipation-related surgery (P=.003) and 1.5% developed Parkinson's disease (P=.019). In multivariate analysis, age 65 years or older at diagnosis of constipation (hazard ratio, 4.834; 95% CI, 2.088-11.190) and presence of STC (hazard ratio, 2.477; 95% CI, 1.046-5.866) were independently associated with development of Parkinson's disease. Only 5 patients had a new diagnosis of CRC during the follow-up period. The risk of CRC did not differ significantly between patients with vs without STC (P=.575).In a long-term follow-up study of patients with constipation in Korea, most had no severe complications. However, patients older than 65 years with a new diagnosis of STC might be considered for Parkinson's disease screening.