The aim was to assess the trends in prescribed defined daily doses (DDD) and drug expenses before and after the introduction of a computerized cost containment module. On January 1993 the module was introduced in 20 practices in Aarhus County that used the computer record system APEX. Two control groups were made. From the Public Health Insurance in Aarhus County (600,000 inhabitants) data were collected during 1992 and 1993. Compared with te controls there were no changes in prescribed DDD, reimbursement for prescribed drugs, and reimbursement per prescribed DDD in the intervention group after the introduction of the module. Cost containment procedures should be more intensive than just giving the doctors a computer-assisted decision aid.