We have assessed short and long-term imprecision goals based on biological variation in haemocytometry through internal quality control (IQC) for the following parameters: white blood cell (WBC), red blood cell (RBC) and platelet (Plt) counts, concentration of haemoglobin (Hb) and mean cell volume (MCV). The attainment of short-term imprecision goals in the IQC system with our own and commercial control blood (between-day control) presents problem for MCV, where minimum performance is achievable, the analytical coefficient of variation (CV) being less than three-quarter of the average within-subject variation (CV I), i.e. CV<0.75 CV I. For the other four parameters desirable performance (CV<0.50 CV I) and even optimum performance—CV<0.25 CV I (for WBC and Hb) is achievable. The long-term desirable imprecision goals (CV L) based on within- and between-subject variation (CV G) seem to be too loose. However optimum performance % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % WGdbGaamOvamaaBaaaleaacaWGmbaabeaakiabgYda8iaaicdacaGG % UaGaaGOmaiaaiwdadaWadaqaaiaadoeacaWGwbWaa0baaSqaaiaadM % eaaeaacaaIYaaaaOGaey4kaSIaam4qaiaadAfadaqhaaWcbaGaam4r % aaqaaiaaikdaaaaakiaawUfacaGLDbaadaahaaWcbeqaamaalyaaba % GaaGymaaqaaiaaikdaaaaaaaGccaGLOaGaayzkaaaaaa!4968! $$ {\left( {CV_{L} < 0.25{\left[ {CV^{2}_{I} + CV^{2}_{G} } \right]}^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} } \right)} $$ is attainable for all five basic haemocytometry parameters. In the IQC system with retained patient specimens (within-day control), short-term imprecision goals for optimum performance are surpassed for WBC and Hb and desirable performance is achieved for RBC, MCV and Plt.