Myeloid leukaemoid reactions were observed in 18 Black adults and 20 Black children at Baragwanath Hospital, Johannesburg, during a 10-month period. This represented an incidence of 1,3/1 000 adult medical admissions and 3,7/1 000 paediatric medical admissions. During the same period the incidence of myeloid leukaemoid reactions in White children in Johannesburg and in Black and White children in Buffalo, NY, was less than 1/1 000 admissions. The reactions were associated with bacterial infections, neoplasms, hepatorenal failure, acute metabolic disorders and non-malignant blood dyscrasias. The mortality in adults was 55,6% and in children 35%. Sixty-five per cent of all deaths occurred within 24--48 hours of admission. Residual morbidity in survivors was common. The pattern of acute leukaemia in the same hospital population is predominantly myelocytic in both adults and children.