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 alpha/beta ratio showed that these findings result from interaction of Hb S with the alpha-thalassaemia gene. Peak C with mean for Hb S of 40% consisted of a majority of individuals with no thalassaemia gene, and the alpha/beta ratio was 0.98% +/- 0.06. Peak B individuals were heterozygous for alpha-thalassaemia 2 with an alpha/beta ratio of 0.80% +/- 0.06 and peak A individuals were homozygous to alpha-thalassaemia 2 with an alpha/beta ratio of 0.58 +/- 0.08. The presence of alpha-thalassaemia 2 in homozygotes results in hypochromia and microcytosis, with little effect on haematological parameters.