We studied the hair of a 10-month-old girl who was suffering from acrodermatitis enteropathica, using light and polarizing microscopy before and after institution of zinc therapy. The hair was very thin and brittle. On light microscopy the shafts showed uneven diameter and some displayed atypical trichorrhexis nodosa with stretched fractures. Ten percent of the hair fibers exhibited nodal swellings of the pseudomonilethrix type. Polarization microscopy disclosed in 70% of all hair shafts an irregular pattern of alternating dark and bright bands. This anomaly was still present in 10% of the hair shafts after one and one-half years of zinc therapy, but could no longer be detected after two years of zinc supplementation. Repeated determinations of hair probes before and after treatment gave a low cystine content, however, being still in the normal range. We assume that the observed changes and the low hair cystine content can be attributed to the underlying zinc deficiency.