The epidemiology of dyspepsia is reviewed with reference to the factors that affect prevalence, including definitions of the term, case mix, and selection. Period prevalence of dyspepsia in several different populations gives an average of 32%, of which 24% is accounted for by recognized ulcer disease. Dyspepsia appears to comprise about 70% of patients' gastrointestinal problems in a large prospective survey carried out in general practice in England, higher than some other estimates. The proportion of gastrointestinal disease in general practice consultations is examined, and while it accounts for about 5% of all consultations, it accounts for about 14% of patients consulting with a problem. There seems to have been a decline of 15% over 30 years. The difficulty of establishing the population prevalence of functional dyspepsia is emphasized, and several studies in which the proportion in general gastrointestinal outpatients has been measured are reviewed. Some data from a Glasgow study in which clinical histories have been recorded directly from patients by a computer system (GLADYS) show the prevalence of several common gastrointestinal symptoms in a clinic population and also of dysmotility-like dyspepsia. Such descriptive data should also be used for predicting diagnosis and for selecting patients to investigate.