Bioaerosol characteristics were evaluated in hospital clean rooms with different class levels. For total particles, an airborne particle counter was used to determine the particle size ranges (0.1, 0.2, 0.3, 0.5, 1 and 5 microm) for air inlets and patient beds. An Andersen 1-STG sampler was used for bacterial and fungal collection. For aerosol characteristics, it was found that some air inlet particle levels were higher than 100000/foot(3) in class 100000 clean rooms. In addition, it was clearly demonstrated that particle concentrations in patient beds were much higher than those for air inlets. Human activity might play a role in these particle concentration differences. Moreover, it was demonstrated that bacterial and fungal concentrations ranged from 1 to 423 and from 0 to 319 CFU/m(3), respectively. For class 100 clean rooms, no particles were ever found. In addition, bacterial concentrations were found to be in the range of 0-32 CFU/m(3) and there were no fungal aerosols. For operating rooms of class 10000, some of the particle levels observed were higher than 10000/foot(3). Furthermore, the average level of bacterial aerosols was 88 with a range of 13-336 CFU/m(3). In addition, fungal levels ranged from 0 to 51 with a mean value of 4 CFU/m(3). It was indicated that bacterial levels were higher than fungal ones, which might be related to human sources. Moreover, there were weak relationships among class level, particle concentration and bioaerosol levels.