One hundred consecutive patients with an ESR of 100 mm or more in the first hour admitted to a general medical unit were studied. Their mean age was 67 years and forty-seven were male. Three patients recovered without a satisfactory diagnosis. In thirty-three of the remainder a single diagnosis was considered responsible for the elevation of the ESR, and in the others multiple diagnoses were found. Infection was found in 60% of patients, malignancy in 28% (including 7% with myelomatosis), rheumatoid disease in 20% and renal disease in 11%. 34% of patients died within 6 months of entry into the study. In the absence of rheumatoid disease or a paraproteinaemia, elevation of the ESR in excess of 60 mm in the first hour at 1 month or longer was associated with a particularly poor prognosis. This study has shown the diagnostic implications of an ESR of 100 mm or more in the first hour and the prognostic significance of a persistent elevation of the ESR.