Compound heterozygotes for sickle haemoglobin (HbS) and hereditary persistence of fetal haemoglobin (HPFH) have high fetal haemoglobin (HbF) levels but few, if any, sickle cell disease-related complications. We studied 30 cases of HbS-HPFH (types 1 and 2), confirmed by molecular analysis, and report the haematological features and change in HbF levels over time. These results were compared to those of patients with sickle cell anaemia or HbS-β0 thalassaemia, including a subgroup of patients carrying the XmnI polymorphism, known to be associated with elevated HbF. Among the HbS-HPFH patients, HbF level was 50–90% during infancy and declined steeply within the first few years of life, stabilizing between ages 3 and 5 years, at approximately 30%. Mean HbF of individuals age 5 or older was 31 ± 3%, average haemoglobin concentration was 130 ± 10 g/l and average mean corpuscular volume (MCV) was 75 ± 4 fl. Univariate and multivariate regression analyses significantly associated HbF with age, haemoglobin concentration, and MCV (P < 0·001). There was a strong inverse association between HbF and age (r = −0·9, P < 0·001). Despite having a much higher HbF level, patients with HbS-HPFH have a similar age-related pattern of HbF decline and associations as patients with sickle cell anaemia or HbS-β0 thalassaemia.