Three athletes, one cross-country skier and two middle distance runners, experiencing poor response to intensified training and decline in performance during prolonged periods of frequent competitions, associated with a tendency to develop anaemia, were found to have spherocytosis, and increased osmotic fragility of the red blood cells. All had family members with the same blood abnormality, but without symptoms. The observations suggest that the frequency of hereditary spherocytosis is higher than indicated by overtly affected cases and that mild cases are easily overlooked. Hereditary spherocytosis should be looked for in athletes with a tendency to decline in blood haemoglobin concentration during periods of intensive training and competition.