Amylose has a small but significant number of long-chain branches. Two-dimensional size-exclusion chromatography is used to obtain the first measurement of the average number of branches per amylose molecule (from potato tubers) as a function of molecular size. Molecular weight dispersity, average chain length and average amylose molecular weight all increase with increasing size. However, the average number of branches of amylose molecules is weakly, if at all, dependent on size, with 2-4 per molecule except perhaps for the very largest molecules, although for these, the data may suffer from artifacts. Differences in the sizes of amylose molecules is mostly ascribed to variations in chain length. This observation is consistent with the postulate that most branching events occur in the early stage of amylose synthesis, and afterwards the branches are further elongated by granule-bound starch synthases. This gives improved mechanistic understanding of amylose biosynthesis.