Rhodium(III) porphyrin complexes [Rh(Por)Me] are catalytically active towards stereoselectiveintramolecular carbene C–H insertions of α-diazoacetamides to give cis-β-lactams or trans-γ-lactams in yields up to 99% with regioselectivities up to 100% and cis/trans ratios up to 83:17 for β-lactams.