[en] The present report contains the results of the third phase of an experimental investigation concerning frictional pressure gradients for flow of boiling water in vertical channels. The test section for this phase consisted of an electric heated stainless steel tube of 3120 mm length and 3.94 mm inner diameter. Data were obtained for pressures between 8 and 41 ata, steam qualities between 0 and 58 %, flow rates between 0.0075 and 0.048 kg/sec and surface heat flux between 20 and 83 W/cm. The results are in excellent agreement with our earlier data for flow in 9.93 and 7.76 mm inner diameter ducts which were presented in reports AE-69 and AE-70. The present measurements substantiate our earlier conclusion that the non dimensional pressure gradient ratio, ψ² , is, in the range investigated, independent of mass flow rate, inlet subcooling and surface heat flux. On the basis of the measured pressure gradients, the following empirical equation has been established for engineering use: ψ² = 1 + 2400(x/p)^0.96 This equation correlates our data (about 800 points) with a discrepancy less than ± 15 per cent and is identical with the corresponding equation obtained from measurements with the 7.76 mm duct