Eight unrelated patients with Hunter syndrome were investigated for expression of iduronate-2-sulfatase (IDS) mRNA by reverse transcription (RT) linked to polymerase chain reaction (PCR), or RT-PCR. The entire coding region was studied by amplification of two overlapping segments of 0.7 and 1.1 kb. Seven children with Hunter syndrome had PCR products indistinguishable in size from normal. One patient, with clinically severe disease, did not produce either IDS product although mRNA for a control gene was readily amplified. This method rapidly identifies patients having absent or qualitatively abnormal IDS mRNA and may be useful in investigating genotype-phenotype relationships.