A comparison between the α- and β-facial selectivity observed in the hydroboration of some androst-5-enes and B-norandrost-5-enes does not parallel the difference between the calculated force field energies for α- and β-cyclobutane models suggesting that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial π-complex between the alkene and the borane.