The influence of humidity on changes to the pressure drop of flat and pleated HEPA filters clogged by polydisperse hygroscopic and non-hygroscopic aerosols has been studied. For flat filters, the results confirm the observations reported by Gupta et al. (1993) that with a hygroscopic aerosol at humidity below the deliquescent point, or with a non-hygroscopic aerosol, the particulate specific cake resistance decreases as relative humidity increases. For pleated filters as for flat filters, the results reveal that during the formation of the particulate cake, the increase in relative humidity leads to a decrease in the specific cake resistance. When the reduction in filtration area, specific to pleated filters, becomes large, the presence of humidity accelerates the filling of pleats causing a greater pressure drop for a same aerosol mass loading. With a hygroscopic aerosol at a humidity above its deliquescent point, the change in pressure drop through flat or pleated filters is not linear but characteristic of liquid aerosol filtration. The influence of humidity on the efficiency of pleated filters, measured by a soda fluorescein aerosol, has also been studied. For clean filters, the efficiency decreases with increasing relative humidity above 90%. For filters clogged with solid aerosol, change in efficiency versus collected surface mass reaches a maximum whose value depends on the relative humidity; when the aerosol is liquid, the efficiency decreases throughout the clogging.