The C:N ratio of the pharmaceutical wastewaters is usually suitable for a combination of the anaerobic pretreatment with the high COD removal and aerobic posttreatment with the efficient biological N removal. This kind of anaerobic-aerobic process was tested in semipilot scale by using a UASB reactor and an activated sludge system with a predenitrification (total volume 100 l). It was found that at a total HRT of 2.3 days an average of 97.5% of COD and 73.5% of total N was removed. The UASB reactor was operated at 30 degrees C with a volumetric loading rate of 8.7 kg.m-3.d-1, the efficiency of COD removal was 92.2%. The processes, which take part in the biological removal of nitrogen, especially the nitrification, were running with lower rates than usually observed in aerobic treatment systems.