INTRODUCTION Discrete roughness elements occur in straight pipes for various reasons. For example@ systematically distributed roughness elements are often incorporated on a pipe wall to promote turbulence and enhance heat transfer. Discrete roughness elements@ such as welds at pipe joints@ occur naturally due to manufacturing processes. Also@ there is evidence that dissolved minerals in fluids form deposits which usually have a certain amount of visual regularity about them. This can occur in boiler tubes when the fluid becomes saturated. In all cases there is a significant increase in friction pressure drop over that due to a nominally smooth pipe. Item No. 660272 deals with friction losses in hydraulically-smooth straight pipes and in pipes with randomly distributed roughness such as that due to surface finish. This Item deals with friction losses in straight pipes with systematically distributed roughness elements of certain geometric configurations on the pipe wall. The types of roughness element considered are (i) transverse@ two-dimensional ribs (Section 6)@ (ii) helical threads (Section 7)@ (iii) longitudinal or spiral fins (Section 8)@ (iv) three-dimensional arrays (Section 9)@ (a) hemispherical projections (Section 9.1)@ (b) truncated pyramids (Section 9.2). In the main part of the Item isothermal friction factor data are given for pipes of circular section containing all the preceding types of roughness element. For pipes of non-circular section fewer data have been found but tentative guidance is given in Appendix A for pipes of rectangular section containing ribs and for concentric annuli containing ribs or threads. As@ in many situations@ roughness elements are employed to improve heat transfer@ a tentative correction is applied to the isothermal friction factor to allow for the corresponding cross-sectional changes in the fluid physical property values (see Section 5).