Objective. In a cystometry procedure in a child with myelomeningocele (MMC), a pressure increase in the abdominal pressure (P-abd) tracing was detected during filling. This pressure alteration was not related to other known events (straining, talking, rectal contractions). This study was conducted to investigate this phenomenon. Material and methods. Forty-three children with MMC were enrolled in the study. A slow and gradual pressure increase associated with the bladder filling was sought in the P-abd tracings. End filling and initial P-abd gradient more than 3 cm H2O were considered as increased P-abd. If the defined pressure event occurs, the bladder was evacuated for verifying the filling-pressure relation. Age, gender, study position, pelvic floor tonicity and cystometric capacity were correlated with the pressure alteration. Results. P-abd increase was noted in 18 (41.8%) children. The mean P-abd gradient between end and initial filling was 4.78 +/- 1.63 cm H2O in these children. No statistically significant difference was noted for age, gender and study position. Statistically significant differences were noted with decreased pelvic floor tonicity and high values of cystometric capacity (p = 0.003 and p < 0.001, respectively). Conclusions. The pressure increase is thought to be a consequence of a posterior positional change in the bladder during filling die to decreased pelvic floor support in MMC. This pressure alteration was more obvious with increased bladder capacity. Urodynamic studies of children with MMC should be carefully evaluated for the presence of this phenomenon to prevent low measurement of the detrusor pressure, compliance and detrusor leak point pressure values.