The effect of topically applied active vitamin D3 on epidermal regeneration was studied in eight healthy volunteers following sellotape stripping. The mobilization of resting (Go) cells was assessed by means of DNA flow cytometry, using a narrow window in the mid-S phase to monitor the progress of the recruited cells. Although vitamin D3 was applied at a concentration exceeding the range reported to be effective in psoriasis, we were unable to find any effect of active vitamin D3 on Go mobilization in vivo.