The sickling disorders are a common cause of morbidity and mortality in Jamaica. Sickle cell beta+-thalassaemia is the fourth commonest form, occurring in one in every 3000 births. This is a heterogeneous condition, producing HbS, HbF and HbA2 with variable amounts of HbA, depending on the mutation and, within a defined population, only a few beta-thalassaemia mutations occur at high frequency. This study establishes the frequency of beta-thalassaemia mutations in Sbeta+-thalassaemia patients in Jamaica. In addition, comparison of the haematological phenotypes is possible by looking at the 'average steady-state haematology' of the different mutational groups. Blood samples from 132 unrelated Sbeta+-thalassaemia patients attending the MRC Sickle Cell Unit at the University of the West Indies were analysed by amplification refractory mutation system (ARMS) polymerase chain reaction (PCR) or sequencing to determine the nature and frequencies of the underlying beta-thalassaemia mutations. Ten mutations were identified, four of which accounted for 93% of patients studied. These were -29(A --> G) in 71 (54%), -88(C --> T) in 27 (20%), polyA(T --> C) in 17 (13%) and IVS1-5(G --> C) in nine (7%). The six remaining mutations found at low frequency were C24(T --> A) in two patients and one each of IVS2-848(C --> A), -90(C --> T), IVS1-5(G --> T), IVS1-5(G --> A) and IVS1-6 (T --> C). In one individual, no mutation was found. The three commonest mutations were all associated with haemoglobin levels of greater than 10 g/dl, whereas IVS1-5 (G --> C) had a more severe haematological phenotype. The predominance of -29(A --> G) and -88(C --> T) is in keeping with other studies on populations of African origin. IVS1-5(G --> C) is found chiefly in Indian populations, and all affected families acknowledged Indian ancestry, reflecting the prominent Indian community in Jamaica.