To investigate possible etiologic influences on the development of constipation, the present study examines the concordant occurrence of constipation with other diseases. Using 8.8 million Medicare patients hospitalized in the United States during 1987, the frequency distribution of all three-digit International Classification of Diseases (ICD) codes was compared in constipated patients and patients without constipation. Most associations and the closest associations were observed between constipation and neurologic diseases, the majority of which resulted from damage to the central nervous system. Associations between constipation and disorders of the spine also involved neuronal damage. The influence of potential confounding factors such as immobility or pain on the development of constipation was assessed and found to be minimal. The frequent association of constipation with disorders involving neuronal malfunction suggests that disruption of the neural modulation of colonic motility may play an important role in the development of constipation.