BACKGROUND: An automatic procedure to detect and recognize the specified drugs for a target controlled infusion system based on the absorption ratio of the optical signals (red and infrard light) was proposed. METHODS: A red (660 nm) and infrared (925 nm) light emitting diode (R-IR LED) and photodiode was established at two perforated holes on a 50 ml syringe horizontally and opposite. The syringe was isolated from extra-light sources, such as sun-light or fluorescent light, with a dark sheet. First, the light of the R-IR LED was emitted into free space without insertion of any other syringe. After measuring the intensity of radiation, a 30 ml syringe containing a drug solution was pushed into the 50 ml syringe and again the intensity of light was measured. We measured intensities of light for 1% or 2% propofol and 0.5% thiopental sodium three times and compared it to the measurement of the free space. In order to investigate mechanical error, we measured it again two days later. We calculated errors among each measurement, simple errors and ratios between first and second time measurements and whether the error was within a permissible error range (less than 1%). After all of this, we evaluated the accuracy and clinical usefulness of this new method. RESULTS: Ratio of intensities of transmission of light in each drug solution to free space showed thiopental, 1% propofol and 2% propofol in order. Considering permissible error between the first and second time measurements, this method showed no problem in recognition of kinds and concentrations of drugs. CONCLUSIONS: We supposed that this simple optical method for drug recognition can be applied for target controlled infusion operation instead of the currently available magnetic recognition tag combined to a syringe.