The number of lil mutant mice that survived to birth was less than 5% of total progeny, rather than the 25% expected for a recessive mutation. We evaluated litters at different embryonic time points to determine whether the reduced number was due to intra-uterine demise. Litters were collected at E12.5, 13.5, 15.5, 17.5, and 18.5, and embryos were genotyped and evaluated for evidence of intra-uterine demise, including growth retardation, pallor, and tissue friability. lil embryos had a progressively higher rate of demise between E13.5 and E15.5. At E12.5, 20 out of 87 (23%) were homozygous for the mutation, and all of these appeared viable. At E13.5, 13 of 49 (27%) were mutant and two had died. At E15.5, 22 out of 91 (24%) were mutant and the majority of mutant embryos (17 out of the 22) had died. By E18.5, nine of 29 embryos (31%) were homozygous for the mutation, of which only one embryo was viable. Diaphragmatic muscularization was abnormal in all mutant mice examined (n > 25). Pulmonary hypoplasia was observed in 100% of mutants evaluated for that phenotype between E11.5 and birth (n > 50).