Some laboratory investigations are abnormal during the course of Crohn's disease (CD). We investigated the trend of some of these laboratory tests in a group of patients with CD to study the relationships between an activity index made up of such laboratory parameters only (LCDAI) and the usual Crohn's disease activity index (CDAI). One hundred thirty-one examinations of 63 patients were evaluated. At each investigation, besides calculation of the CDAI, 10 laboratory investigations were carried out. Three gastroenterologists independently gave an overall evaluation of the laboratory activity for each of the 131 examinations on the basis of the results of the blood tests alone. The sum of the evaluations was used as an independent variable on which a laboratory index was developed by multiple regression analysis. C reactive protein, red cell sedimentation rate, acid alpha 1-glycoprotein, alpha 1-antitrypsin, and white blood cells had an important share in the development of this laboratory index. The evaluation of the relationships existing between LCDAI and CDAI showed that in patients with moderate to severe clinical disease activity, LCDAI was constantly altered. The same happened in 55% of cases in clinical remission, which suggests an inflammatory activity that is not clinically evident. These results point to the advisability of supplementing a predominantly clinical index, such as CDAI, with a laboratory index such as LCDAI in the evaluation of CD.