Pressure drop has been determined experimentally for downward co-current annular two-phase flow, with water and air flow Reynolds numbers in the ranges 5100-27200 and 3400-21600, respectively. In order to investigate the effect of tube diameter, four tubes, of I.D.'s ranging from 1.56-4.12 cm, were tested. Special care was taken to eliminate effects of surfactants. The resulting skin friction coefficients were only in qualitative agreement with most previously reported data. At a fixed gas Reynolds number, a large increase in friction accompanied a decrease in tube diameter. Existing correlations were found to be unsuccessful for the present data.