Comparison of the results obtained with medium-fill water cystometry and rapid-fill CO2 cystometry were done in 102 consecutive patients. In 99 patients the cystometric diagnoses were identical. In 3 patients we found detrusor hyperreflexia with water but a normal cystometrogram with CO2. We concluded that CO2 cystometry gives as valid information as water cystometry. Volume at first desire to void, bladder capacity and bladder compliance is approximately 50% higher in water cystometry than in CO2 cystometry in normal cystometries. In cases of detrusor hyperreflexia there were no important differences in bladder volume at first uninhibited detrusor contraction or in highest intra-vesical pressure during detrusor contraction.