Blood samples from 21 patients with confirmed hereditary spherocytosis (HS) were studied using the Technicon H1 blood counter. Nine patients had had a splenectomy and 12 were unsplenectomized. In the latter, the means of MCHC and RDW as measured by the H1 were significantly higher than results obtained using the Technicon H6000. Furthermore, spherocytes were recognizable in all of the H1 histograms of red cell size and Hb concentration as distinctive hyperchromic and microcytic tails, and the HDW (Hb distribution width) was consistently elevated. In the splenectomized patients, although the mean MCHC and RDW results were significantly higher as measured by the H1, it was not possible to recognize any abnormalities in the red cell histograms, and the HDW results were normal. Two of 75 apparently normal blood donors showed very small tails of hyperchromic cells, but in neither was this sufficient to disturb the MCHC, RDW or HDW parameters, and confusion with an unsplenectomized case of HS would be very unlikely. The H1 appearances of HS are so specific that the measurement of osmotic fragility will seldom be necessary.