There are few data about the distribution of neutralizing antibodies (NA) against adenovirus types in the Italian population, especially the high-numbered ones. We tested the sera from 453 children and 51 young adults to evaluate NA against adenovirus prototypes 1-33. Using the microneutralization test, 338 (74.6%) of the children's sera were positive for at least one adenovirus type. Antibody to type 2 was the most frequently detected followed, in descending order, by antibody to types 5, 1 and 3. All these types are known to be associated with disease but antibody to type 7, a type also associated with disease, was less frequent than that to other serotypes such as 18 and 31, the pathogenicity of which in man is not clearly established. The antibody positivity rate rose with age for the more frequent types while it did not vary for the less frequent ones. The number of sera with NA against more than one adenovirus type increased with age. With regard to types 1-8, we found that their frequencies in Italy were similar to those found in the U.S.A.