ESDU 88001 provides an empirically derived procedure for predicting the ratio of the critical heat flux in a horizontal tube to that in upward flow in a vertical tube under the same conditions (which may be calculated by the method of ESDU 86032). A databank (also available from ESDU) was created from all the experimental results in the literature for horizontal flow and was used to develop a correlation for the variation of the ratio of the critical heat fluxes with mass flow rate and flow quality. It contained 900 points, and the reasons for discarding a further 500 are discussed. There was considerable scatter and recommendations are made for a safe prediction if required as well as the use of the mean of the results. The accuracy of the prediction is assessed. Two detailed worked examples illustrate the use of the procedure. Its application to non-uniformly heated tubes is considered, and the effect of an upstream bend is also discussed.