Pressure driven flow of an incompressible Newtonian fluid in a spiral duct of square cross-section was studied both experimentally and numerically. The duct has a curvature ratio (Rc=R/a, where R is the radius of curvature and a is the duct dimension) of 15.1 at the inlet and spirals inwards for nine turns at a uniform rate. A one-component laser-Doppler anemometer was used to measure streamwise velocities. The flow development was determined for Dean number, Dn, of 100, 125, 150, 180 and 250. based on the radius at the flow inlet [Dn = Re/(Rc)1/2, where Re is the Reynolds number, v′θa/ν]. Steady oscillations in the streamwise direction between 2-cell and 4-cell states, first predicted by Sankar et al. [Phys. Fluids 31, 1348 (1988)], were observed for Dean numbers between 139 and 240. No time dependent flow phenomena were observed. The experimental data are in very good agreement with the numerical simulations, which were based on the parabolized steady three-dimensional Navier-Stokes equations. The results are consistent with calculations by Winters [J. Fluid Mech. 180, 343 (1987)] that predict the existence of a region where no stable two-dimensional solutions exist.