A sudden change in how area induces complex transient processes in foams, which increase in significance with increasing flowrate. The effects are a function of the method of foam generation and, hence, depend on the initial foam structure. In vertical upflow at a sudden expansion or contraction, a substantial fraction of the liquid in the foam drains back upstream. This leads to a greatly reduced liquid holdup downstream and a recirculating flow regime upstream. For the area ratios of the fittings investigated, the extent of liquid holdup reduction was less pronounced in the case of a sudden contraction. At low gas rates, a plug flow regime usually existed on either side of the fitting; however, with an increase in gas throughput the presence of the fitting led to the establishment of a recirculating flow regime upstream. At sufficiently high flowrates complete breakdown of foam structure occurred resulting in the onset of a 'flooding' regime downstream. Foam rheology was successfully described by a two-parameter power-law model, and the friction factor for foam flow could be described by a simple explicit relationship.