The diagnostic clarification of joint effusions of unknown origin is a challenge to every primary-care physician. Important diagnostic procedures are arthrocentesis and analysis of the aspirated synovial fluid. Synovial fluid analysis frequently allows differentiation between harmless effusions due to osteoarthritis and crystal induced inflammation, or the more devastating septic arthritis. 4475 synovial fluids were evaluated retrospectively to calculate the identification rate of crystals compatible with calcium pyrophosphate dihydrate (CPPD) and monosodium urate monohydrate (MSUM). 40.8% (1827) of synovial fluids were taken from females and 59.2% (2648) from males. The frequency of crystal identification varied considerably: 13.2% CPPD crystal identification in females, 10.9% in males; MSUM was identified in 1.5% of females, and in 10.9% of males. The spectrum of joint involvement was nearly identical in CPPD and MSUM positive synovial fluids. Exceptions were the higher frequency of CPPD identification in shoulder joints (CCPD:MSUM = 15.6:1), the higher frequency of MSUM identification in the ankle (MSUM:CPPD = 15.6:1) and the first metatarsophalangeal joints (MSUM:CPPD = 8:1). Clinical suspicion correlated well with crystal identification in MSUM positive samples (60%), but was poor in CPPD positive samples (36%). The poor correlation between clinical suspicion and crystal identification in CPPD positive synovial fluids is explicable by the less characteristic clinical presentation of pyrophosphate arthropathy in contrast to classical gout. A high percentage of crystal identification was found in joints or periarticular swellings in which aspiration is difficult and therefore rare (e.g. tendon sheaths, first metatarsophalangeal and first metacarpophalangeal joints), underlining the importance of synovial fluid aspiration despite the difficulty of arthrocentesis.