The threshold properties of photonic crystal quantum dot lasers operating in the slow-light regime are investigated experimentally and theoretically. Measurements show that, in contrast to conventional lasers, the threshold gain attains a minimum value for a specific cavity length. The experimental results are explained by an analytical theory for the laser threshold that takes into account the effects of slow light and random disorder due to unavoidable fabrication imperfections. Longer lasers are found to operate deeper into the slow-light region, leading to a trade-off between slow-light induced reduction of the mirror loss and slow-light enhancement of disorder-induced losses.PACS numbers: 42.55. Tv, 42.70.Qs, Slow light in photonic crystal (PhC) line-defect waveguides [1] enhances the interaction between the propagating light wave and the material of the waveguide, and has enabled the demonstration of increased material nonlinearity [2], enhanced spontaneous emission into the propagating mode [3,4], and enhanced material gain [5]. Such engineering of fundamental materials properties is important for the development of integrated photonic circuits, with applications in classical as well as quantum information technology. Microcavity lasers can be realized in the same PhC membrane structure by exploiting highquality point-defect cavities and in the past decade significant progress was made [6][7][8], culminating in recent demonstrations of high-speed electrically pumped structures [9]. Such PhC lasers allow the exploration of new operation regimes, such as single emitter lasing [10] and ultra-high speed modulation [11]. However, while it was shown that slow light in combination with random spatial disorder leads to very rich physics [12][13][14][15][16][17][18], the role of slow light on lasers realized using defect cavities has apparently not been systematically investigated. For the case of passive point-defect cavities, it is well known that disorder is an important factor limiting the quality factor [19][20][21][22] but the role of slow light in extended active cavities is not well understood.In this paper, we report experimental results on PhC quantum dot lasers with variable cavity length and show that these attain a minimum threshold gain for a certain cavity length, in stark contrast to conventional lasers, where the threshold gain decreases monotonically with cavity length. We derive a rate equation including the effect of slow-light propagation and show that the experimental observations may be explained when taking into account disorder-induced losses. These results show that disorder may lead to fundamental limitations on the performance of nanostructured lasers, but the results also demonstrate a promising platform for investigating disorder effects in active structures, such as the competition between deterministic cavity modes and random modes formed by Anderson localization [18]. . The PhC structure has a lattice constant of a = 438 nm and an air-hole radius of 0.25a. A so-called LN cavity [25...