An outbreak of occupational dermatitis in an electroforming plant where there was heavy exposure to nickel is described. Patch test investigations confirmed nickel allergy in 13 of 27 exposed individuals. Nickel chloride was found to be a more reliable patch test allergen than nickel sulphate. Improvements in industrial hygiene led to an immediate decrease in the incidence of dermatitis. Persistent patch test sensitivity to nickel was found in three individuals who had been removed from the process chemicals for some years.