Hemifusion, the merger between two opposing membrane leaflets, is considered a key step for fusion pore formation. However, the model of fusion through hemifusion has not yet been directly demonstrated for exocytosis in living cells. Using high resolution darkfield and fluorescence microscopy in alveolar tape II pneumocytes containing large (> 1 mm) secretory vesicles (lamellar bodies), we show that lamellar body fusion events with the plasma membrane were accompanied by a mostly bi-phasic scattered (darkfield) light intensity decrease (SLID) originating from the border of the fusing lamellar body. Correlation of SLID with the diffusional behaviour of various fluorescence markers for either content or membrane mixing revealed that the onset of the fast, second, phase of SLID corresponded to the formation of a fusion pore, followed by lamellar body swelling. In contrast, the slow, first, phase of SLID preceded pore formation but could still be accompanied by diffusion of farnesylated DsRed, a marker for the inner plasma membrane leaflet, or nile red. We conclude that the first phase of SLID reflects the hemifusion state of exocytotic lamellar body fusion, while the second phase reflects fusion pore opening, fluid entry and lamellar body swelling. These data indicate that exocytotic fusion proceeds through hemifusion intermediates. Furthermore, hemifusion may last up to 8 s. SLID is a highly sensitive and non-invasive approach by which hemifusion can be detected independently of membrane markers, thereby providing a new tool to investigate the regulation of hemifusion in living cells.