The 1-azaspirocyclic ring system can be found in a number of bioactive natural product families. Lepadiformine, cephalotaxine and cylindricine A all share a common spirocyclic pyrrolidine core, and fasicularin, halichlorine, histrionicotoxin and cylindricine B possess a homologous spirocyclic piperidine centre. This paper describes a concise synthesis of spiropyrrolidines and spiropiperidines which employs a ruthenium–alkylidene-catalysed cross-metathesis reaction of enantiopure N-protected allylglycine with methylenecycloalkanes. The resultant alkene intermediates can then undergo a tandem acid-catalysed cyclisation to form spiropyrrolidines. Ring expansion of the spiropyrrolidine system, via an aziridinium intermediate, grants access to the homologous spiropiperidine ring system with excellent stereo-retention.