The detection and reporting of serious adverse drug reactions (SADRs) have become important components of monitoring and evaluation activities performed in hospitals. We present the implementation of a prospective pharmacovigilance program based on automatic laboratory signals (ALSs) at a hospital. We also report the general findings after the first year of operation of the program, which involved ALSs that indicate various SADRs: agranulocytosis, aplastic anemia, liver injury, thrombocytopenia, hyponatremia, and rhabdomyolysis. The number of hospitalizations during the year was 54,525, and 1,732 patients experienced at least one ALS. The review of electronic medical records (EMRs) showed that no alternative cause (i.e., no non-SADR explanation) for the ALS was identified in 520 (30%) of the patients. After the individual ALS-patient evaluation, a total of 110 SADRs (6.35% of those identified after reviewing EMRs and 21.15% of those requiring individual patient evaluations) were identified. In other words, in order to identify a single SADR, we had to review the electronic records of approximately 16 patients and personally visit 5 patients.