BACKGROUND: Prolonged slow expiration (PSE) is a manual chest physical therapy technique routinely performed in clinical practice. However, the reliability and agreement of the technique have not been tested. OBJECTIVE: The objective of this study was to assess reliability and agreement between physical therapists during the application of PSE in infants with wheezing. DESIGN: This was a cross-sectional study. METHODS: Infants with a mean age of 59 weeks (SD = 26 weeks) were included in this study. Two physical therapists (physical therapist 1 and physical therapist 2) randomly performed 3 PSE sequences (A, B, and C). The expiratory reserve volume (ERV) was measured with a pneumotachograph connected to a face mask. ERV was used to evaluate the reproducibility of the technique between sequences and between physical therapist 1 and physical therapist 2. RESULTS: The mean ERV of the infants was 63 mL (SD = 21 mL). There was no statistically significant difference between the ERV values in the 3 sequences for physical therapist 1 (A: mean = 46.6 mL [SD = 17.8 mL]; B: mean = 45.7 mL [SD = 19.9 mL]; C: mean = 53.3 mL [SD = 26.3 mL]) and physical therapist 2 (A: mean = 43.5 mL [SD = 15.4 mL]; B: mean = 43.2 mL [SD = 18.3 mL]; C: mean = 44.8 mL [SD = 25.0 mL]). There was excellent reliability between the sequences for physical therapist 1 (ICC = 0.88 [95% CI = 0.63-0.95]) and physical therapist 2 (ICC = 0.82 [95% CI = 0.48-0.93]). Moderate agreement was observed between physical therapist 1 and physical therapist 2 (ICC = 0.67 [95% CI = 0.01-0.88]). According to Bland-Altman analysis, the mean difference between physical therapist 1 and physical therapist 2 was 4.1 mL (95% CI = -38.5 to 46.5 mL). LIMITATIONS: The data were collected in infants with wheezing who were not in crisis. This decreased lung mucus; however, it also reduced evaluation risks. CONCLUSIONS: PSE was a reproducible chest physical therapy technique between physical therapists.