More than 40 orthovoltage X-ray therapy units are monitored by calibrated thermoluminescent dosimeters (TLD). Three pairs of LiF:Mg,Ti and CaSO4:Dy pellets are irradiated in air in a polypropane capsule. The range between 0.2 to 3.0 mm Cu the half-value layer (HVL) is determined from the CaSO4-LiF ratio. In this range, the response of the LiF dosimeters is corrected for energy dependence. We are using a fixed sensitivity for LiF in the MVL range of 2 mm Al to 0.2 mm Cu. In the first two years 173 monitorings were performed by using 485 badges. Usually not more then two different radiation qualities (in the HVL ranges 2 mm Al to 0.2 mm Cu and 0.9-2.5 mm Cu) or two beams used on a therapy unit are measured by using 2-3 badges per monitoring. In 134 cases (77.5%) the dose rates were within acceptable limits (+/- 10%). In 23 cases (13.3%) a failure of the X-ray unit was detected. We think that our system is simple enough and may be used for the dosimetric monitoring of orthovoltage X-ray therapy units. In some cases even the type of malfunction can be determined.