Transcripts of the lysosomal cysteine proteinase cathepsin L are spliced into five variants (L-A, L-A1, LA-2, LA-3 and L-B), which have similar stabilities but different translation efficiencies, thus potentially yielding diverse amounts of the protein. The aim of this study was to investigate whether the abnormally elevated expression of cathepsin L in invasive tumours is due to overexpression of L-A3, the splice variant translated most efficiently. The expression pattern of cathepsin L mRNA variants was measured by quantitative polymerase chain reaction (qPCR) in two panels of cell lines obtained from precancerous and cancerous breast tissue. In both panels, the cell line exhibiting the highest in vitro invasiveness also expressed the highest amount of L-A3. Although the pattern of expression varied, the expression of the L-B variant was always remarkably lower than for other variants. We propose that posttranscriptional regulation of cathepsin L expression is altered during breast tumour progression.