Increased spontaneous contractile activity in detrusor muscle may contribute to overactive bladder contractions and increased bladder tone associated with urinary incontinence¹. The underlying mechanisms for enhanced contractile function in overactive bladders have never been examined, in particular, the role of intracellular Ca²⁺, which critically determines contractile function. The objective of this investigation was to test the hypothesis that enhanced spontaneous Ca²⁺ oscillations may be associated with detrusor overactivity. Human detrusor samples were obtained with ethical approval and patient consent. Dissociated detrusor myocytes were loaded with Fura-2 or Fluo-4; whole cell and local Ca²⁺ concentration ([Ca²⁺]_i) was measured using either conventional epifluorescence microscopy or a laser-scanning confocal system. The magnitudes of whole cell spontaneous [Ca²⁺]_i rises were quantified as the time-averaged area under the curve for the duration of observation. Parameters for quantifying Ca²⁺sparks were amplitude (F/F0), full duration at half-maximum (FDHM, ms) and full-width at half-maximum (FWHM, µm). Data are mean±s.e.m. Student’s t-test examined differences between data sets (p<0.05). Spontaneous transient global rises of [Ca²⁺]_i were observed of varying amplitude, duration and frequency and associated with cell shortening. They were Ca²⁺-dependent and sensitive to the L-type Ca²⁺-channel antagonist verapamil. The mean increases of [Ca²⁺]_i were 5.7±1.2 nM and 12.8±2.3 nM in cells from stable (n=24) and overactive bladders respectively (n=26, p<0.05). Localised rises of [Ca²⁺]_i, as discrete, fast sparks and larger cluster-like local activities were also observed in unstimulated myocytes. These localised sparks either remained local or spread to nearby areas. Repetitive firing of Ca²⁺ sparks and their fusion usually progressed into Ca²⁺ waves, leading to whole cell [Ca²⁺]_i transients. Spark activity was inhibited by 10-20 μM ryanodine and facilitated by 1 mM caffeine. The intensity, duration and width of local Ca²⁺ activity were increased in overactive bladders (F/F0 1.53±0.05 vs 1.32±0.02, FDHM 776±153 vs 358±83 ms, FWHM 4.46±0.33 vs 2.55±0.17 µm; n=53 sparks, 4 overactive bladders; n=103 sparks, 5 stable bladders, p<0.01). These observations show that global and localised Ca²⁺ rises can occur spontaneously in resting human detrusor myocytes. Whilst whole cell Ca²⁺ rises generate spontaneous contractile activity, the local Ca²⁺ oscillating events, via ryanodine receptors, contribute to a global Ca²⁺ rise. The upregulation of both global and local Ca²⁺ activities in detrusor myocytes from overactive bladders will contribute to increased contractile activity in overactive bladders.