Heterogeneity in the expression levels of mammalian genes is large even in clonal populations and has phenotypic consequences. Alternative splicing is a fundamental aspect of gene expression, yet its contribution to heterogeneity is unknown. Here, we use single-molecule imaging to characterize the cell-to-cell variability in mRNA isoform ratios for two endogenous genes, CAPRIN1 and MKNK2. We show that isoform variability in non-transformed, diploid cells is remarkably close to the minimum possible given the stochastic nature of individual splicing events, while variability in HeLa cells is considerably higher. Analysis of the potential sources of isoform ratio heterogeneity indicates that a difference in the control over splicing factor activity is one origin of this increase. Our imaging approach also visualizes non-alternatively spliced mRNA and active transcription sites, and yields spatial information regarding the relationship between splicing and transcription. Together, our work demonstrates that mammalian cells minimize fluctuations in mRNA isoform ratios by tightly regulating the splicing machinery.