Introduction  Progressive macular hypomelanosis (PMH), a condition of uncertain etiology, is characterized by asymptomatic hypopigmented macules predominantly located on the trunk. To date, there are no reports from South‐East Asia concerning this condition. We sought to record the clinical features of PMH in Asian patients, identify etiologic factors, and study the structural and ultrastructural features of melanocytes in this disorder.

Methods  Patients who presented to the National Skin Center with acquired, hypopigmented macules on the trunk, without a history of inflammation or infection, were recruited. Erythrocyte sedimentation rate (ESR), complete blood count, fasting blood glucose, liver function tests, skin scrapings for fungi, and skin biopsy specimens (from lesional and normal skin) were obtained. Biopsies were stained with hematoxylin and eosin (H&E), Fontana Masson, an immunohistochemical panel for identification of melanocyte differentiation antibodies (HMB 45, Melan A, and S100) and CD 68. Electron microscopy (EM) was also performed. The patients were evaluated every 3 months.

Results  During a 9 month period, eight patients (all Chinese) presented with hypopigmented, ill‐defined, confluent macules involving the lower aspect of the trunk. There were four men and four women, and the mean age was 25.9 years (range 19–45 years). Skin scrapings were negative for fungi and laboratory tests were normal.

Microscopic evaluation of skin biopsy speciments showed reduced pigmentation of lesional as compared with normal appearing skin, but H&E‐stained sections revealed only minimal histologic differences between lesional and normal skin. EM demonstrated a statistically significant (P = 0.047, Wilcoxon Signed Rank Test, Wilcoxon 95% CI 0.02–0.62) higher ratio of stage IV and late stage III (dark) melanosomes in normal vs. lesional skin.

Conclusions  PMH may occur among young adults in Singapore. Its etiology is uncertain. The melanin content of lesional skin appears to be less than that in normal sites. EM shows a higher ratio of immature melanosomes in lesional vs. normal skin.