Pulpal response to marginal enamel etching or both enamel and dentine etching with 37% phosphoric acid for adhesive composite restorations was studied in the teeth of the Japanese monkey (Macaca fuscata). Dentine etching irritated the pulp and caused moderate to severe initial changes along the odontoblastic layer at 3 days. When no bacteria were present along the cavity walls, the subsequent pulpal reactions at 30 and 90 days decreased with time and large amounts of irritation dentine were formed. These inflammatory reactions were less than those caused by enamel etching only, or zinc oxide/eugenol cement. Significant correlation (P less than 0.01) was found between the intensity of the inflammatory reactions and the degree of bacterial infection. The pulpal irritation caused by acid etching of dentine for composite restorations is transitory only when the fillings have a good marginal seal and wall adaptation.