The cholinesterase genotypes in the majority (25/35) of patients with suxamethonium sensitivity following termination of pregnancy are heterozygotes with an E1a gene. Twelve of these patients have the genotype E1uE1a. The reported duration of apnoea is minimal in the heterozygotes lacking the E1a gene (about 5-10 min) and maximal in the homozygotes E1aE1a (about 35 min). With few exceptions, the heterozygotes having an E1a gene are apnoeic for 10-15 min. The apparent low frequency of suxamethonium in these patients is discussed.