The aim of this study was to determine the growth rate of onion plants and the content of dry matter and selected organic compounds in common onions during the growing season. Field experiments were conducted in 2012-2013 in the Kujawy-Pomerania Province (53°6′50″N, 19°4′6″E). The experimental materials comprised onion plants of four cultivars: Błońska, Dakota F₁, Norvito F₁ and Octavia. Seeds were sown in the middle of April. Between 20^th June and 5^th September 30 onion plants were selected randomly for an analysis of the morphological traits of bulbs and leaves. The following parameters were determined: the weight, horizontal diameter and vertical diameter of bulbs, the number of leaves involved in photosynthesis, and the concentrations of dry matter, total sugars, reducing sugars and L-ascorbic acid. In the group of the common onion cultivars under analysis, onion plants of cv. Norvito F₁ were characterised by the highest increase in the weight, horizontal diameter and vertical diameter of bulbs, and the number of leaves per plant in the experimental period. The content of dry matter and total sugars in onions increased throughout the growing season. On the last sampling date, onion plants of cv. Norvito F₁ had the lowest dry matter content and contained the highest concentrations of sugars.