SummaryExamination of 24 case reports suggests the worm burden in those presenting to hospital with intestinal obstruction is >60 and is about tenfold higher in fatal cases.keywords Ascaris lumbricoides, ascariasis, intestinal obstruction, worm burden, morbidity, mortality correspondence Dr N. R. de Silva, Department of Parasitology, Faculty of Medicine, University of Kelaniya, Talagolla Road, Ragana, Sri Lanka It is generally accepted that a 'large' number of worms must necessarily be present in the small intestine in order to cause obstruction. However, the number meant by 'large' seems somewhat uncertain: figures mentioned in this context range from 15 to more than 100 (Baird et al. 1986; Thein-Hlaing et al. 1990;O'Hanley & Pool 1995;Gilles 1996). Better knowledge of the average worm burden in patients with Ascaris-induced intestinal obstruction could be of predictive value in calculating morbidity rates due to acute complications of ascariasis (Chan et al. 1994).An exhaustive search failed to reveal any reports in the mainstream literature on worm burden in an identified series of patients. We then looked at data given in case reports, noting the total worm burden (as counted at surgery, autopsy, or after an anthelmintic), age and outcome (survival/death). Only 24 case reports (Watson 1920;Baugh 1923;Alles 1924;Levin & Porter 1924;Watkins & Moss 1924;Hsu et al. 1940;Jenkins & Beach 1954;Aiken & Dickman 1957;Cole 1965;Crane et al. 1965;Gandhi 1965;Piggott et al. 1970; BustamenteSarabia et al. 1979;Rathi et al. 1981;Katz et al. 1985;Baird et al. 1986; Mpairwe 1991) which specified all of the above were found among 53 publications which described a total of more than 1000 patients.Nine of the 24 cases were fatal. The worm burden in these 9 (median 639, mean 659.2, range 23-1978) was significantly higher than in the non-fatal cases (median 59, mean 219.3, range 4-990, P=0.045, Kruskal-Wallis H-test) (Figure 1) . Four of the 9 fatal cases and 9 of the 15 non-fatal were below Figure 1 Comparison of worm burden in fatal (n=9) and non-fatal (n=15) cases, and in under-fives (n=13) and older patients (n=11). Vertical line indicates range, box indicates 25th and 75th percentiles. , Median; , mean.