This study was conducted in the Eastern Province of Saudi Arabia where both thalassaemia and Hb S genes occur at a high frequency. In 171 Saudi Hb S heterozygotes, the range for Hb S was found to be 17–45% with a mean of 31.0%. Frequency distribution histograms showed a trimodal distribution. Peak A, B and C had 18–28, 28–35 and 35–45% Hb S values, respectively. Determination of α/β ratio showed that these findings result from interaction of Hb S with the α-thalassaemia gene. Peak C with mean for Hb S of 40% consisted of a majority of individuals with no thalassaemia gene, and the α/β ratio was 0.98% ± 0.06. Peak B individuals were heterozygous for α-thalassaemia 2 with an α/β ratio of 0.80%) ± 0.06 and peak A individuals were homozygous to α-thalassaemia 2 with an α/β ratio of 0.58 ± 0.08. The presence of α-thalassaemia 2 in homozygotes results in hypochromia and microcytosis, with little effect on haematological parameters.