Thirty-two subjects who suffered from dandruff participated in a study in which one-half of the head was washed with a shampoo containing 1% zinc pyrithione (ZPT) and the other half was washed with the same shampoo without ZPT. Four groups, eight subjects per group, were shampooed one, three, six or nine times (shampoo frequency twice per week). Clinical dandruff gradings of each half of the head were made 4 days after the last shampoo in each group, when scalp biopsy samples were also taken from each half of the head. Measurements of labelling index (LI), mean epidermal thickness (MET), and assessment of the numbers of PAS- and Gram-positive micro-organisms were made on the biopsy samples. There was a progressive reduction in dandruff on the sides of the head treated with the ZPT shampoo, the differences relative to the placebo-treated areas being statistically significant after three, six and nine washes. There were no significant differences in LI between treatment groups and the MET was shown to vary according to the treatment and the number of washes. There was a significant reduction in the number of PAS-positive micro-organisms (but not Gram-positive micro-organisms) on the ZPT-treated areas.