In the first study conducted to investigate the nutritional value of raw and extruded narrow-leafed lupin (Lupinus angustifolius) seeds cv. Boruta, 60 male Ross 308 chickens of age 16–20 days were used (Experiment 1). In 35-day performance trial (Experiment 2), 960 1-day-old chickens were randomly allotted to 11 treatments and fed diets containing 0, 50, 100, 200, 250 or 300 g · kg⁻¹ diet of raw or extruded narrow-leafed lupin seeds. In the first experiment, extrusion of narrow-leafed lupin seeds led to the decrease of neutral detergent fibre (NDF) concentration. Seed processing increased fat digestibility and nitrogen retention in chickens but had no effect on ileal digestibility of protein and amino acids. There was a tendency to increase the apparent metabolizable energy corrected to zero N balance (AME_N) after lupin extrusion. In the second experiment, the inclusion of increasing levels of raw or extruded seeds into diets quadratically decreased body weight gains (BWGs) of birds, except for broilers fed extruded lupin on days 15–35. The extrusion increased the BWGs of birds in the whole trial period (days 0–35). Increasing levels of raw and extruded narrow-leafed lupin seeds quadratically increased feed conversion ratio during the experiment. So, the extrusion of narrow-leafed lupin seeds had a limited positive effect on the chemical composition of seeds and growth performance of broiler chickens.