Using data from a Danish dietary validity study, we evaluated the influence of including individually estimated portion size data on the validity of a semiquantitative food frequency questionnaire. A total of 144 subjects, aged 40-64 years were included. Correlation coefficients and classification of subjects into quintiles according to their intake of foods and nutrients were used to compare questionnaire data, with and without individually estimated portion size data, with data obtained by 2 x 7 days weighed diet records. For men, the mean correlation coefficient for food group comparisons was 0.47 when information about individually estimated portion sizes for food items that do not come in natural units was included in the analysis. Using a common average portion size reduced the mean correlation coefficient to 0.45. For women, similar changes were observed (0.36 to 0.35). For nutrients the mean values for the comparison changed from 0.51 to 0.49 and from 0.39 to 0.40 for men and women, respectively. For both men and women the classification into quintiles according to their intake of foods and nutrients showed only minor differences. We conclude that little extra information was gained by including individual portion size information for food items that do not come in natural units. This may reflect that portion sizes are of minor importance compared with frequencies or that the relevant individual portion sizes were not estimated correctly.