Interestingly, we found that two patients had complex thalassemia syndrome: one had co-inheritance of compound heterozygous HbCS with -α3.7, having genotype -α3.7/αCSα, βE/β−28, and the other had co-inheritance of compound heterozygous HbCS with α-thalassemia 1 (–SEA/αCSα, βE/β−28), diagnosed hematologically as CSEFABart’s disease, an uncommon form of thalassemia intermedia rarely observed in the Thai population. The gene discussed is BCS1L; the disease is beta-thalassemia intermedia.