We describe a case of classic trichothiodystrophy occurring as an isolated disorder. This is the second reported case of trichothiodystrophy unassociated with systemic disorders, in contradistinction to other cases where there have been profound associated neuro-ectodermal abnormalities. The hair has shown the pathognomonic polarizing light-microscopic findings and the sulphur content has been confirmed to be reduced to approximately 50% by energy dispersive X-ray micro-analysis. Formal light-microscopic studies of hair cross-sectional shape using a Hardy microtome have confirmed the previous finding of a very thin or absent cuticle. An additional previously unreported finding was a 'scalloped' contour in a majority of hairs. The previously described ribbon shape could not be confirmed. Follow-up observation of the patient has shown clinical improvement with time.