Haemoglobin H/Constant Spring is an important cause of severe haemoglobin H disease, but the Constant Spring protein is difficult to detect by electrophoresis. A technique for allele specific polymerase chain amplification of the 3'-end of the α2 globin gene improved detection of the αCSα haemloglobin variant in DNA samples by slot-blot hybridisation. The αCSα mutation was confirmed in subjects that had been previously diagnosed by haemoglobin electrophoresis, and it was also detected in patients who were negative by protein electrophoresis. 10 of 103 unrelated Laotians with HbE were αCSα heterozygotes. Of these, 3 were negative to the normal probe because they had -α37/αCSα with a single α globin deletion. 5 samples did not amplify or hybridise to either probe because they had deletions of both α2 globin regions. The gene frequency for αCSα is about 0.05 for Laotians. This technique, which is highly specific and sensitive for rapid detection of the αCSα mutation, is suitable for clinical diagnoses and population studies. The true incidence of αCSα may prove to be greater than previously suspected from protein electrophoresis.