In an empiric approach to develop the definition of persistent diarrhea, we evaluated the relationship between diarrheal duration and risk of ensuing clinically significant decline in nutritional status, in a cohort of 395 children < 24 mo. Weights were obtained at the onset of diarrhea (wt I) and after three months interval (wt II). The occurrence of an adverse outcome (AO) was defined as a decline of -- 5% in NCHS weight for age (% WFA) between weights I and II or death in this interval. The risk of AO was similar for episodes of / or > 7 days while it was substantially higher in episodes with > 14 days duration (45%) than for shorter duration episodes, relative risk (RR) = 2.5 (p < 0.001). Relative risk remained similar for duration thresholds of 21 (2.3) and 28 days (2.6). As episode durations greater than 14 days are associated with substantial elevation of the risk of clinically cogent sequelae, such episodes may be termed 'persistent' at least in terms of poor prognostic expectations.