Background It is unclear whether hair washing is effective against scalp eruption and pruritus caused by seborrheic dermatitis or psoriasis vulgaris.

Aims To assess whether a proper hair‐washing regimen, including the use of antibacterial shampoo, can ameliorate scalp eruption symptoms and alter the composition of the scalp microflora.

Methods Eighteen patients with seborrheic dermatitis or psoriasis vulgaris scalp eruptions were instructed in proper techniques of daily hair washing, rinsing, and shampooing, which they underwent for 12 weeks. They used control shampoo in weeks 1–4 and 9–12, and an antibacterial shampoo during weeks 5–8. At the start of the test period and at weeks 4, 8, and 12, we assessed scalp symptoms (erythema, scaling/desquamation, dryness, itchiness, and scratching scars); microbial DNA levels from lesion and nonlesion areas; and levels of interleukin (IL)‐1α, IL‐1ra, and total protein in the scalp's horny layer.

Results Compared to baseline values, scaling/desquamation and itchiness improved significantly at weeks 8 and 12. Other observed skin symptoms also improved over time. Malassezia colonization levels in lesion and nonlesion areas decreased gradually; the decrease was significant at week 8 in lesion areas and at weeks 4 and 8 in nonlesion areas. Bacterial colonization levels also decreased gradually, achieving significance in lesion areas at week 4. Gradual decreases in IL‐1ra/IL‐1α level showed statistical significance at weeks 4 and 12, while the protein quantity significantly decreased at week 12.