Optically pumped InGaN/GaN quantum well (QW) vertical-external-cavity surface-emitting laser operating at 420 nm wavelength with external cavity length of up to 50 mm has been realized. Use of injection seeding enables us to reach lasing operation even for pump pulse duration just slightly exceeding the external cavity roundtrip time. The carrier lifetime in the QWs was measured to be 2.5 ns, and lasing emission was obtained under pump pulses shorter (400 ps) or longer (10 ns) than the carrier lifetime in QWs. We studied experimentally the dependence of the lasing threshold on the length of external cavity and provided a model to explain this dependence. Further detailed measurements of the near field and far field patterns, optical spectrum, spontaneous emission factor, and delay time to emission of the lasing pulse confirm lasing at the external cavity mode. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789806]