Protein hydrolysates (PHs) are added to hair-care products (to "repair" broken hair), soaps, bath gels, creams, etc. From one to 22 PHs used in hair-care products (collagen, keratin, elastin, milk, wheat, almond, and silk) were tested in three patient groups: A) 11 hairdressers with hand dermatitis B) 2160 consecutive adults with suspected allergic respiratory disease subjected to routine skin prick tests C) 28 adults with atopic dermatitis. In group A, all the 22 PHs were tested with scratch and patch tests. In groups B and C, one to three PHs were tested with prick tests. Positive scratch/prick test reactions were seen in 12 patients from three PHs altogether. All were women with atopic dermatitis, and all reacted to at least hydroxypropyl trimonium hydrolyzed collagen (Crotein Q). In three patients, prick and open tests with a hair conditioner containing Crotein Q were performed with positive results. One patient reported contact urticaria on her hands, and two reported acute urticaria on their head, face, and upper body from a hair conditioner containing Crotein Q. In seven of the eight studied sera, specific IgE to Crotein Q was detected. In conclusion, PHs of hair cosmetics can cause contact urticaria, especially in patients with atopic dermatitis.