Formation of agglomerates of calcium oxalate monohydrate (COM) crystals on semi-batch precipitation peformed at conditions relevant to urolithiasis (37°C, pH = 6, initial ratio [Ca]/[Ox] = 10), but without any specific admixture, was followed by both optical and electron microscopy. COM crystals formed on precipitation developed into large agglomerates consisting of intergrown crystals by a mechanism of primary agglomeration. Primary agglomeration of COM crystals represents an important mechanism of COM renal calculi growth