The purpose of the present study was to develop a method for short-term caries trials in man, using an experimental model already employed in periodontal research. Twelve students were given prophylaxis and instruction to practice meticulous oral hygiene until Plaque Index and Gingival Index approached zero. The carious state of the bucco-gingival enamel areas, excluding molars, was evaluated using a dissection microscope. The cleaned and dried tooth surfaces were scored according to the Caries Index system. During a period of 23 days without oral hygiene 6 participants performed 9 daily mouthrinses with 10 ml of a 50% sucrose solution. At the termination of this period the sucrose group demonstrated a higher mean Caries Index and a greater number of new early lesions than the six control subjects. The mean Plaque Index scores were higher in the sucrose group, whereas the Gingival Index scores were similar for both groups. Smears of plaque showed no significant differences in the percentage distribution of various types of microorganisms. After 30 days of oral hygiene and daily mouthrinses with 0.2 % NaF, fair agreement was found between the Caries Index scores of the two groups. Similar observations were made after a second hygiene and fluoride period. The results demonstrate that it is possible to produce detectable carious alterations within 23 days by abolishment of oral hygiene and frequent sucrose additions and that these alterations are reversible.