A new test facility was designed and constructed to simulate flow in a horizontal well with a single perforation. A total of 635 tests were conducted with Reynolds numbers ranging from 5000 to 60,000 with influx to main pow rate ratios ranging from 1/5 to 1/100, and also for the no-influx case. The pow behavior in a single-perforation horizontal well was investigated both theoretically and experimentally. A general new friction factor expression for a single-perforation horizontal well was developed. A new simple correlation for the horizontal well friction factor,vas developed by applying experimental data to the general friction factor expression. The new friction factor correlation and experimental data were compared, with the Asheim et al. (1992) data and model, and showed that the new correlation performed better than the Asheim et al. (1992) model.