Crystal arthritides such as gout can be detected by ultrasonography (US). This study reveals the performance of joint US (double contour sign (DCS), tophus (T), hyperechoic spots cq. "snow storm" (SS)) for diagnosing gout and calcium pyrophosphate dihydrate crystal deposition disease (CPPD) in patients with acute mono- or oligoarthritis (MOA). The gold standard is the presence of monosodium urate (MSU)/CPPD crystals. Fifty-four Dutch patients had an acute MOA. US was performed on the following six joints maximum: the arthritic joint, the contra lateral side, metatarsophalangeal (MTP)-1, and knees bilaterally in case of arthritis in one of these joints. In case of wrist/PIP/MCP-arthritis, the knees and MTP-1 were scanned. These were examined for DCS, T, SS, and intercartilage rim (CPPD). Synovial fluid was aspirated from the affected joint for MSU proof. Twenty-six of the 54 (48 %) patients with MOA had MSU-proven gout. Sensitivity of DCS and any US abnormality (DCS, T, SS) was 77 and 96 %, respectively. The positive likelihood ratio (LR+) for DCS and any ultrasonographic abnormality (USabn) was 3.08 and 2.99, respectively, and the LR- was 0.31 and 0.06, respectively. In MSU-proven gout patients where the affected joint is not MTP-1, MTP-1 still showed USabn in 42 % of the patients. None of the CPPD patients had an intercartilage rim. In dedicated hands, ultrasonography deserves a place early in a screening algorithm of MOA patients, particularly if specificity is high enough to make punctures abundant or when microscopy is not available. In 86 % of the MSU-proven gout patients, the DCS is not present in another joint other than the affected or MTP-1 joint.