Histological features of early mycosis fungoides (MF) can simulate numerous inflammatory lesions and histological confirmation of
MF is often delayed, compared with clinical diagnosis. Recently, using molecular techniques, the detection of a dominant T-lymphocyte clone has been reported in cutaneous lesions of MF. The aim of the present study was to determine the diagnostic value of a dominant T-lymphocyte clone as assessed by PCR-DGGE in early MF. Histopathological and molecular analyses were performed on cutaneous lesions from 104 patients clinically suspected as having MF. In this population, the positive predictive value of a PCR (+) was 0•86. In addition, four of six patients whose lesions were PCR (+) (detectable dominant T-cell clone) but not histologically MF progressed to MF within 2-48 months. In order to evaluate the relevance of PCR -DGGE in MF follow-up, serial biopsies were performed in 24 patients. In 89 per cent of cases, the presence or absence of a PCR (+) was constant during the course of the disease. When present, the DGGE imprint of PCR products was case-specific. These data demonstrate the diagnostic value in MF of T-lymphocyte clonality assessed by PCR -DGGE on cutaneous lesions and show that the technique can be used in MF follow-up to evaluate residual disease with high specificity. 1998 John Wiley & Sons, Ltd.