Modulation of actin isoform expression is a well-established feature of developmental phenomena. As one might expect, it is also characteristic of several pathological situations that are the subject of the present review. alpha-Smooth muscle actin has proven to be a reliable marker for identifying (a) vascular smooth muscle cells during vascular development and vascular diseases, and (b) myofibroblasts during wound healing, fibrocontractive diseases, and stromal reaction to epithelial tumours. The hallmark of a differentiated myofibroblast relies on the acquisition of an organized contractile apparatus characterized by alpha-smooth muscle actin-expressing stress fibres. More and more data suggest that alpha-smooth muscle actin plays a direct role in myofibroblast contractile activity through its N-terminal domain AcEEED. Newly developed antibodies against alpha-skeletal and alpha-cardiac actins have allowed the detection of subpopulations of alpha-skeletal positive cardiomyocytes in adult, hypertrophic, and failing heart. These antibodies have also permitted us to identify the differentiation degree of malignant cells in tumours such as rhabdomyosarcoma. Whether the differential expression of actin isoforms in human diseases is functionally relevant is not yet fully established, although studies on human actin mutations, actin null mice, and the N-terminal end of alpha-smooth muscle actin support this possibility.