<p>This study presents a numerical investigation of developing and fully developed laminar flow in microtubes whose inner surfaces are patterned with superhydrophobic longitudinal grooves. The effects of gas area fraction and groove–rib spacing on the hydrodynamic entrance length and axial velocity distribution are examined over a range of Reynolds numbers. The numerical model is validated against available theoretical correlations, showing good agreement. The results indicate that the maximum increase in entrance length due to increases in gas area fraction and Reynolds number is approximately 9% and 41%, respectively, demonstrating that the Reynolds number has a more pronounced influence. The increase in entrance length caused by gas area fraction is found to be independent of Reynolds number, while the effect of Reynolds number is similarly independent of gas area fraction. Furthermore, the increase in slip velocity with gas area fraction is shown to be independent of Reynolds number. It is also shown that the decrease in the non-dimensional groove–rib combination causes the slip velocity to decrease, and the maximum slip velocity is lower than the average fluid velocity flowing through the microtube.</p>