The present study was carried out to determine the intracellular free calcium concentration ([Ca 2+ ] i ) and the activity of its regulatory enzymes (Na + ,K + -ATPase and Ca 2+ -ATPase) in leucocytes. Levels of plasma lysophosphatidylcholine (LPC) were also measured. Then the relationship between these parameters and the clinical severity of asthma and bronchial reactivity was studied. Patients with asthma were divided into three groups: acute asthma (subjects in acute exacerbation), uncontrolled asthma (subjects currently symptomatic) and stable asthma (subjects currently asymptomatic). A group of normal subjects was also studied. Spirometry, specific airway conductance and bronchial reactivity measurements were carried out. The following biochemical parameters were studied in venous blood: leucocyte [Ca 2+ ] i , Na + ,K + -ATPase and Ca 2+ -ATPase activities, and plasma LPC. Leucocyte [Ca 2+ ] i was increased and the activities of Na + ,K + -ATPase and Ca 2+ -ATPase were decreased in patients with asthma. Plasma levels of LPC were also increased. These changes were observed to be greatest among asthmatics in acute exacerbation of asthma, and lesser in magnitude in patients with less severe asthma. The activities of both ATPases were found to have a significant positive correlation, and [Ca 2+ ] i and the levels of plasma LPC a significant negative correlation, with predicted forced expiratory volume in 1 s (FEV 1 ). No significant correlation was observed between the biochemical parameters and bronchial reactivity. It is concluded that intracellular calcium homoeostasis is abnormal in asthma; specifically, the activities of Na + ,K + -ATPase and Ca 2+ -ATPase are decreased. These abnormalities may modulate the clinical severity of asthma.