Economic reasons are the cause that ever more frequently prosthetic replacements are made from alloys other than gold. The most widely used are: alloys based on nickel and chromium, and stainless steel. In some alloys the content of nickel may be over 80%. The studies carried out in early 1980s showed that these materials had a higher hardness, mechanical strength and elasticity module than gold alloys. Nickel is one of the main components of these alloys, and its compounds may exert a harmful toxic, and even more frequently, allergic effect on the human organism. In the light of a survey of the literature and own clinical, experimental and epidemiological investigations it was tried to trace the effect of dental replacements made of nickel-containing alloys on the mucous membranes and skin of subjects with confirmed hypersensitivity to nickel. The studied material comprised a group of patients selected at random from those attending the Allergology Outpatient Clinic at the Department of Dermatology, Medical Academy in Warsaw, who had verified hypersensitivity to nickel and had indications to preparation of crowns and bridges for wearing. In each case before and after prosthetic treatment and before any control stomatological examination patch tests were done for demonstrating nickel hypersensitivity. In all patients permanent prostheses were done from an allow containing up to 70% of nickel with baked porcelain. In most cases fragments of oral mucosa were taken for histological examination. The patients were examined twice yearly. The longest follow-up is presently over 8 years. In no case exacerbation of hypersensitivity reactions was noted.