The constant supply of broccoli heads for a vegetable market can be kept by the differentiation of sowing time and the age of seedlings. Dark-chilling of young seedlings can also modify the time of transplanting, subsequent plant development, yield and inflorescence quality. The aim of the present study was to analyze the influence of seedling dark-chilling on the inflorescence initiation and the morphological quality of broccoli heads. The experiment was carried out at the University of Agriculture in Krakow (South Poland) in the years 2009 and 2010, with broccoli (Brassica oleracea L. var. italica Plenck) medium late cultivar ‘Monaco’ F1. The examined factors were the age of seedlings (4-, 6-, 8-, and 10-week-old) and dark-chilling (2°C/2 weeks and non-chilled control). Microscopic analysis of the apex sections was performed to determine the time of the inflorescence differentiation in chilled and non-chilled seedlings of different age. Chosen morphological features of the mature heads were also described as depended on age and chilling of seedlings. The inflorescence initiation was observed between 4 and 6 week of the plant ontogeny. Especially 4-week-old seedlings, chilled by two weeks, were more advanced in the generative development in comparison to non-chilled control. Low temperature applied in the seedling stage did not cause the head buttoning. Inflorescences developed by broccoli planted from 4 and 6-week-old seedlings, both chilled and control, were characterized by the best visual quality, determined by a convex head shape, tight branching angle, small and uniform size and regular form of flower buds. Chilling of oldest, 8- and 10-week-old seedlings negatively affected the inflorescence quality, through planed and spread shape of heads and non-uniform shape of flower buds.