An experimental investigations was carried out on a perforated circular cylinder (0.50m diameter) encircling an impermeable cylinder (0.05m diameter) at a constant water depth of 0.7m for both regular and random waves in the wave flume at the Department of Ocean Engineering, Indian Institute of Technology, Madras, India. The porosity of the outer cylinder was varied from 4.54 to 19.15% to study the influence of porosity on wave forces on and water surface fluctuations in and around the twin cylinder system. A numerical method is developed based on the boundary integral equation method along with a porous body boundary condition, where the porosity is modeled using the resistance coefficient f and added mass coefficient Ca for regular waves. The resistance coefficient increases with the increase in porosity and wave heights except for a porosity of 4.54%, whereas the added mass coefficient is almost zero. Porosity in the range of 10-15% can be recommended for the perforated cylinder based on the experimental results of wave forces and the water surface fluctuations inside the perforated cylinder. © 2008 ASCE.