Increased and disorganised expression of CD44 variant exons has been demonstrated in biopsy samples of several types of human malignancy by many groups. This abnormality can be used to detect exfoliated tumour cells in the urine of bladder cancer patients. The present report demonstrates that the deranged activity of this gene in neoplasia also results in the accumulation of immature mRNA transcripts containing non-coding sequences (introns) from this gene in cancer tissues and cells. There is simultaneous overexpression of a newly identified 437 bp exon (coding sequence) located in the variably active region of the gene and of many abnormal variant exon combinations. This is the first report describing the specific retention of introns in gene transcripts in clinical diagnostic samples of tumour tissues and cells. The phenomenon was seen repeatedly in samples from cancer patients and in a cancer cell line, and thus could form the basis for a unique new and specific method of cancer diagnosis.