The aim was to compare the effects of two sodium lauryl sulphate (SLS)-containing toothpaste formulations with and without betaine on human oral mucosa in vivo. The results are compared with the effect of a dry mouth toothpaste without SLS. Twenty subjects participated in the double blind, crossover study. The toothpastes with 1.2% SLS, 1.2% SLS and 4% betaine and with 4% betaine but not with SLS were placed on buccal mucosa in a test chamber and kept in place for 15 min. The condition of the mucosa was studied both visually and using electrical impedance (EI) for up to 45 min. Both SLS-containing pastes had a similar, irritating effect on the mucosa as judged both by the appearance of the mucosa and the EI measurements. The dry mouth toothpaste (with betaine only) showed no significant irritation of the mucosa. Betaine did not reduce the mucosa-irritating effect of the SLS-containing toothpaste formulation. The surfactant-free toothpaste did thus not irritate the human oral mucosa in vivo while the SLS-containing pastes did.