Amyloidoses are a group of severe human disorders characterized by deposits of insoluble protein fibrils and severe tissue damage, with Alzheimer's and Parkinson's disease being the most prominent examples. Recent findings suggest that cell toxicity in Alzheimer's diseases is associated, at least in part, with intermediate aggregate populations present during the early phases of fibril formation. Hence, understanding the growth kinetics and structural features of intermediate aggregates is important for unraveling the elusive relation between amyloid deposits and cellular toxicity. We have investigated the early stages of fibril formation of hen egg white lysozyme (HEWL) with dynamic light scattering (DLS) and atomic force microscopy (AFM). HEWL readily forms amyloid fibrils during incubation at acidic pH (< pH 4) and elevated temperature (~50 ºC). Fibril formation was confirmed by the red-shift in the absorption spectrum of Congo Red and by direct visualization of amyloid fibrils with atomic force microscopy. Using DLS, we determined how the size distribution of lysozyme aggregates evolved during the early stages (48 hrs) of fibrillogenesis. Initially the size distribution is monomodal with a mean particle radius slightly larger than that of lysozyme monomers, and with significant peak polydispersity. Both factors suggest the presence of small aggregates from the very outset. For simplicity, we will refer to this peak as "oligomer peak". Following a latency period of several hours a second particle populations sized around 25 nm suddenly emerges. The average radius of these larger aggregates only slowly increases in time, while their relative population increases significantly, and at the expense of the smaller aggregates. Using atomic force microscopy we investigated the detailed morphology of the aggregates present in either peaks. Aliquots of lysozyme were taken at different times during the aggregation process, deposited on mica and imaged with atomic force microscopy in air. Prior to the nucleation of the second peak we find particle sizes and shapes consistent with a mixture of lysozyme monomers and small oligomers. The "nucleation" of the second peak coincides with the appearance of short protofibrils. The heights and width of these protofibrils is very similar to those of the oligomers seen prior to protofibril nucleation. The length of the protofibrils appears to increase in integer multiples of the oligomer size. These observations suggest that HEWL protofibrils not only nucleate from oligomers but also grow via oligomer addition. Supported by an NSF-IGERT and FMMD GRAF scholarship (S.H.) and a Byrd Alzheimer Award (ARG 2007-22) and FCoE BITT Award (M.M).