We have investigated the luminescent properties of highly strained BeZnCdSe quantum wells (QWs). Single-QW structures were grown on (001) n-GaAs substrates by molecular beam epitaxy with various Cd beam pressures. High-resolution X-ray diffraction measurements revealed that the lattice mismatch between the BeZnCdSe QW and the GaAs substrate changed from 2.1% to 3.0% as the Cd beam pressure was increased. The photoluminescence (PL) spectra showed that the peak wavelength was nearly proportional to the lattice mismatch for each sample. Fluorescence microscopy measurements revealed that the PL images of all the samples showed a similar luminescence distribution, but the images of samples having the lattice mismatch of 2.8% and 3.0% showed some dark lines. These results suggest that flat QW layers were formed in all the samples, but the lattice relaxation of the QW layer occurred in highly strained samples with lattice mismatches greater than 2.8%. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)