Alternative splicing involving the inclusion or exclusion of exon 11 in insulin receptor mRNA results in two isoforms of the alpha subunit. The two subunits display tissue-specific variation in relative abundance at both RNA and protein levels and discrete differences in biological properties. We have previously reported a small decrease in the relative level of RNA molecules lacking exon 11 (Ex 11-) in skeletal muscle of non-insulin-dependent diabetes mellitus (NIDDM) patients. In the present study, we describe a drastically altered ratio in favor of Ex 11- RNA in a NIDDM patient with markedly impaired insulin-mediated glucose utilization. The ratio between the splice variants changed from 74% to 48% Ex 11- RNA after initiation of insulin treatment, which considerably improved his blood glucose concentrations and insulin-stimulated glucose utilization rate. This shows that splicing can be regulated by metabolic and/or hormonal factors in response to changes in the in vivo milieu. No genomic deletion or base substitution in either the coding regions or exon-intron borders was found that explains the altered splicing. Heterozygous mutations were excluded in sequences of putative importance for splicing outside the analyzed regions as both alleles were expressed and spliced in an identical fashion. Furthermore, these results suggest that this patient fails to regulate alternative splicing of exon 11 in the manner observed in most NIDDM patients and that this defect is associated with the extreme impairment in insulin action.