Clinical investigation and hair plucking performed during trichogram, a traumatic technique for obtaining a hair formula, are the present means of evaluation in patients with alopecia or complaining of loss of hair. These means do not provide quantitative data valuable enough for a possible appreciation of the severity of hair loss, let alone for a reliable evaluation in therapeutical trials. Phototrichogram, a technique proposed by Saitoh (1970), Fiquet and Courtois (1979) and developed by Bouhanna (1982-1983), brings more satisfactory qualitative and quantitative means of studying hair growth and pilar cycles. Its standardisation had yet to be obtained so as to give results comparable from one study to another and reproducible in any given patient. We propose here a standardised optical equipment and particularly recommend a rigid frame allowing a fixed distance from the skin to the lens, and a superimposed glass window with millimetric graduations; the 0.50 square centimeter skin area to be analysed may easily be marked off by an 0.2 mm wide black microtatoo. This personal technical adaptation gives an easily obtainable definition of five quantitative parameters analysing the hair on a given area of the scalp: density of implantation, telogen percentage, growth rate, mean anagen diameter and percentage of fine hair less than 40 mu in diameter. In the normal male adult our measurements gave the following results on the vertex: density 204 +/- 10 hair per square centimeter, telogen percentage 17.8 +/- 2.8 p. 100, growth rate 0.35 +/- 0.03 mm p. day, mean anagen diameter 76 +/- 5 mu, percentage of fine hair 9.2 +/- 1.8 p. 100.(ABSTRACT TRUNCATED AT 250 WORDS)