Formaldehyde in hospital waste water(10, 000_??_20, 000mg/liter) was decomposed by treatment with slaked lime. The degree of decomposition was dependent on the HCHO and Ca(OH)2 concentrations, reaction temperature and time. Some reducing sugars were newly formed, concomitantly with HCHO decomposition. When hospital waste water with 14, 000mg/liter of HCHO a nd 40, 000mg/liter of COD was allowed to react with 0.5% Ca(OH)2 (30°C for 22hr), HCHO decreased to less than 5mg/liter, and reducing sugars were formed at over 4000mg/liter. On the other hand, COD decreased only slightly by this Ca(OH)2 treatment. The Ca(OH)2-treated formalin waste thus obtained was not only non-toxic to the growth of some tested bacteria, but served as a carbon source for the growth of a Pseudomonas strain. These results suggest that hospital waste water containing fair amounts of HCHO can be made innocuous by the activated-sludge method when pretreated with Ca(OH)2 to decompose HCHO.